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

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.

Add more checks and fix some callers to make sure that the server
core always has a valid screen layout configured.

Do a proper cleanup when one of the termination timeouts trigger
rather than just exiting on the spot. This makes sure we don't leave
stray stuff around, e.g. unix socket files.

We might have an offset in the area of the screen we're tracking,
so make sure that offset is included in the changes we report.

We might only be using part of the X server's screen, so make sure
the layout we calculate is contained within this part.

This makes VNCServer a sufficiently complete interface that callers
don't need to know about the specific implementation (VNCServerST
currently). And assuming that all servers will use sockets is not
that outrageous.

Removed the last parts of VNCSConnectionST's back door in to
VNCServerST and let the parent class fully handle coordination of
clients, and access to the desktop.

Make things simpler by making this a part of the SDesktop interface
that always needs to be implemented.

We cannot assume endian-ness for the cursor we get from XFixes.
Adjust the algorithm to properly follow the specification.

There are some cases where the server state will not automatically
be updated on a change. A prominent one is when only RFB attributes
were changes (e.g. the screen ID) but nothing else. In that case
there is no actual change in the X server, so it never sends any
notification about change back to us.

x0vncserver might only be observing part of the display, so we need
to adjust any coordinates before we give them to the RFB core.

Change Xserver screen through libXrandr. For complex configurations,
all outputs must have corresponding size modes. As a special case, if
the client requests a single screen with an odd size (for example when
adjusting the size of a non-fullscreen vncviewer), find a smaller
suitable mode, and reduce the framebuffer size as well.

It wasn't properly updated in 3112f50 when it was split off from
x0vncserver.cxx.

In XDesktop::start() we allocate pixel buffer and set it as the backend to the given VNCServer.
In XDesktop::stop() we deallocate the buffer, so we must unset it from the VNCServer as well.
Otherwise the VNCServer could try to access it and crash, for example in deferred update.

Legacy systems.

XFixes only reports cursor changes. We must manually call
XFixesGetCursorImage() when starting.

Define ledNames in XDesktop.cxx, rename N_LEDS to XDESKTOP_N_LEDS