Linux implementation using ffmpeg

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.

Without this change I get CMake errors due to use of ${FLTK_INCLUDE_DIR}
and references to the non-existent vncviewer target.

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!

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.

If the difference in angle is larger than 180 degrees we hit a corner
case. This commit modifies the coordinates of an existing test to cover
this.

Makes it easier to understand what the tests do.

It allows us to use the more standard format specifiers.

Allows the user to perform certain important mouse operations using
touch gestures instead.

If you have the setting "Emulate middle mouse button" turned on, a click
and drag can fail if it is done very quickly. The position of the
initial click will be incorrect in such a case because the timeout will
delay events.

Major restructuring of how streams work. Neither input nor output
streams are now blocking. This avoids stalling the rest of the client or
server when a peer is slow or unresponsive.

Note that this puts an extra burden on users of streams to make sure
they are allowed to do their work once the underlying transports are
ready (e.g. monitoring fds).

Just have a simply number of bytes argument to avoid a lot of
complexity.

Makes it more readable to write code that needs to know how much
data/space is available in a stream.

We adapted to the wrong new callback in a previous commit.

The CConnection base class wants to be able to write things these days,
so we need to provide it a place to do so.

Our fast paths assume that each channel fits in to a separate byte.
That means the shift needs to be a multiple of 8. Start actually
checking this so that a client cannot trip us up and possibly cause
incorrect code exection.

Issue found by Pavel Cheremushkin from Kaspersky Lab.

Provides safety against them accidentally becoming negative because
of bugs in the calculations.

Also does the same to CharArray and friends as they were strongly
connection to the stream objects.

Otherwise we might be tricked in to reading and writing things at
incorrect offsets for pixels which ultimately could result in an
attacker writing things to the stack or heap and executing things
they shouldn't.

This only affects the server as the client never uses the pixel
format suggested by th server.

Issue found by Pavel Cheremushkin from Kaspersky Lab.

They have very different purpose, so make things easier to work
with by having multiple directories.

We now filter incoming data, which means we can start assuming the
clipboard data is always null terminated. This allows us to clean
up a lot of the internal handling.

These are not allowed in C++, and have been made optional in C11.
So let's just get rid of them and any issues they may cause.

Avoid using the callbacks used for runtime changes for the initial
setup. They weren't really useful anyway as you could not allocate
a framebuffer without also knowing the pixel format. So make things
more clear by letting serverInit() get the initial settings.

Now that we've split out server state to ServerParams, ConnParams
only contains state for a client. Rename the class and variables
to reflect this.

We need to track different things in the server and client, so
separate things to two independent structures to keep things more
clear.

Avoid direct access to the screen dimensions and layout so that we
can make sure it stays sane. This also makes sure the layout is
properly updated when we only get the screen dimensions from the
server.

We had partial coverage for vncviewer/ already, but fix up the last
pieces.

We use polymorphic exception objects, so catching by value invokes
the copy constructor and stuff that we don't really want.

Bit shifting gives an approximate, but not properly accurate value
when upconverting. Do a proper calculation.

We won't always be on the primary monitor, so check which color space
we're actually using right now. For offscreen stuff we assume a standard
sRGB color space.

This will allow us to use better formats that preserve the entire
alpha channel.

Some platforms draw directly to the screen, which means that updates
will flicker if we draw multiple layers. Prevent this by first
composing the update on a hidden surface.

This will allow us to render more things than just the framebuffer.

Some platforms impose a significant delay per frame, so we need
to separate that from the time spent shuffling pixels.

The complex logic waiting for events didn't result in any added
performance, so use the simpler approach.