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!

It allows us to use the more standard format specifiers.

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.

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.

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