This is a regression from ad0f061. If a VMware cursor rect was split up
over multiple read()s then the stream would become corrupted as we set
the restore point at the wrong place.

The generally recommended way is to include it from source files, not
headers. We had a mix of both. Let's try to be consistent and follow the
recommended way.

Can be helpful to see what encodings were actually used during a
connection, and how they performed.

macOS' built in VNC server unfortunately sends the entire monitor in a
single rect, so we need to be prepared to buffer a lot of data in case
the monitor has a large resolution.

Gives a bit more context where this error happened.

Single CPU machines are extremely rare now, so let's avoid the hassle of
multiple code paths.

The certificates might still be fine using the system trust store, or
the user can make an exception. So let's just log and continue on.

This got very common after 960c7d2 where we now always have a default
value for these settings.

This fixes regression introduced by the extended clipboard extension.
Previously it was possible for the server to hold on to the CLIPBOARD
selection even if another application took ownership of PRIMARY. This is
important to handle the common use case of selecting something in order
to paste over it.

The new request based model doesn't readily support this as we assume
the client has lost its data once we push the new PRIMARY selection to
it. So to handle this we have the maintain a cache of the client's data,
and make sure to fill that cache before we do anything that might cause
the client to lose the data.

The call of gnutls_set_default_priority_append() expects a normal priority
string, which means it must not start with ':'.

Let's avoid making this too complex and force every user to know about
magical functions.

Allows the default value to be dynamically generated in a buffer that is
reused.

Our current architecture doesn't support waiting for a response here, so
don't even try or we'll just get an error.

The session might have died, or failed to initialise properly, so be
prepared for gnutls_bye() to be unable to do its job.

We would leak the security module below the top client VeNCrypt module,
meaning that those modules would not get a chance to shut down
gracefully.

It's perfectly safe to delete NULL pointers, so simplify things by
removing these checks.

Gives us a more meaningful error rather than just "Error in push/pull
function".

Having this early check means that we somewhat randomly get different
exception behaviours on errors in deeper layers as some exceptions are
allowed to propagate unhindered and some are not (since they are thrown
in the pull function).

Give GnuTLS the correct errno from deeper layers, in the cases where we
know it. In most cases GnuTLS doesn't care, but just in case...

CharArray should always be null-terminated. There is a potential
scenario where this all might lead to crash. In Password we call
memset(), passing length of the array we get with strlen(), but
this won't return correct value when the array is not properly
null-terminated.

This change makes it possible for re-synchronizing the remote cursor on
the vncviewer when in fullscreen mode. This is done by locally moving
the cursor position to what the server thinks it should be.

Now SDL games should work!

This change adds support for the VMware Mouse Position
pseudo-encoding[1], which is used to notify VNC clients when X11 clients
call `XWarpPointer()`[2]. This function is called by SDL (and other
similar libraries)  when they detect that the server does not support
native relative motion, like some RFB clients.

With this, RFB clients can choose to adjust the local cursor position
under certain circumstances to match what the server has set. For
instance, if pointer lock has been enabled on the client's machine and
the cursor is not being drawn locally, the local position of the cursor
is irrelevant, so the RFB client can use what the server sends as the
canonical absolute position of the cursor. This ultimately enables the
possibility of games (especially FPS games) to behave how users expect
(if the clients implement the corresponding change).

Part of: #619

1: https://github.com/rfbproto/rfbproto/blob/master/rfbproto.rst#vmware-cursor-position-pseudo-encoding
2: https://tronche.com/gui/x/xlib/input/XWarpPointer.html
3: https://hg.libsdl.org/SDL/file/28e3b60e2131/src/events/SDL_mouse.c#l804

This reverts commit d6e39658ae. Apparently
this is broken on macOS so it's not something we can make use of.

Some of these were incorrectly calculated so the server or client would
wait too long before proceeding with decoding.

Change all of these to be a more explicit calculation to avoid such
issues in the future.

TCP_CORK is a Linux thing and BSD has TCP_NOPUSH instead.

We didn't include the proper headers to get the correct define, so
corking was never enabled.

We computed a safe area if a client gave us a bogus one, but we didn't
actually use it. Fix this properly and make sure we don't pass on bad
coordinates further.

So the current clipboard state is properly reflected in the desktop
session.

This was out of sync with the client handling for no good reason.

The peer expects a response, so we should also be able to respond that
there is no clipboard data currently available.

The extended clipboard protocol has the ability for the peer to request
things to be sent automatically, without a request message. Make sure we
honor such settings.

Each character is more than one byte, so adjust the clearing of the
buffer to reflect that.

Some code points are reserved for the UTF-16 coding itself and must not
appear as input data to the algorithm.

Signed bug prevented anything not ASCII from being coded correctly.

Everything outside of BMP was handled incorrectly and was coded as
completely different code points.

This would mess up most conversions from UTF-8 as the caller wouldn't
know how far to step to get to the next valid character, resulting in
markers for invalid data to be injected here and there.

Also add some unit tests to avoid this reoccurring.

It should be using the safe wrappers for everything so make sure it
cannot bypass those and call the SConnection methods directly.

We incorrectly called the underlying functions instead of the safe
wrappers for the new clipboard functions. This had the effect of a)
crashing the entire server if one of these functions failed, and b) not
respecting the settings disabling the clipboard.

We handled this in the failure scenario, but not in the vastly more
common successful case.

64x64 changed block can be large for fine changes such as cursor
movement and typing in terminal windows, or an update to a clock.

If the block can be efficiently cropped, this will reduce latency
and bandwidth. Every pixel cropped is a pixel less to analyze, encode,
transmit, and decode.

The previous code already detected the top of the change in order
to determine if the block had changed. However, it did not use
this information to reduce the size of the change rectangle, nor
did it calculate any of the other edges.

The new code introduces detection of the other edges, and uses
the information to build a reduced area change rectangle. This
has the additional effect of reducing the number of discrete pixel
values in the change block which may allow a more efficient
encoding algorithm to be selected.

As this section of code is performance sensitive, the method
of detecting the edges has been optimized to quickly fall back
to pessimistic values as soon as a single comparison fails on
each edge. In the case that full 64x64 block are changing,
there will be three extra comparisons per block.

In cases where the change rectangle can be reduced from 64x64,
the reduced size of the change rectangle represents reduced
effort to encode, transfer, and decode the contained pixels.

In the case of images with high frequency changes, which
specifically includes text, the lossy JPEG encoding can be
highly distorted, especially with JPEG level 6 or less. The
quick flash from a distorted JPEG to a lossless JPEG can
appear as a flickering to some people. This effect was more
obvious when the surrounding area is not expected to change,
but is being distorted anyways due to being part of the 64x64
blocking algorithm.

In the case of a user typing in a terminal window, this change
may commonly reduce the number of pixels updated with every
character typed from 4096 pixels (64x64) to 640 pixels (32x20)
or less.