vnc: Add support for color map

vnc: Add support for color map

Our current VNC code does not handle color maps (aka non-true-color) at all
and aborts if a client requests them. There are 2 major issues with this:

 1) A VNC viewer on an 8-bit X11 system may request color maps
 2) RealVNC _always_ starts requesting color maps, then moves on to full color

In order to support these 2 use cases, let's just create a fake color map
that covers exactly our normal true color 8 bit color space. That way we don't
lose anything over a client that wants true color.

Reported-by: Sascha Wehnert <[email protected]>
Signed-off-by: Alexander Graf <[email protected]>
Signed-off-by: Pavel Butsykin <[email protected]>
Signed-off-by: Denis V. Lunev <[email protected]>
Message-id: 1464099559[email protected]

Actually this is a very old patch originally submitted in 2013 by
Alexander. The situation is still the same with RealVNC, it does not
connect by default to QEMU VNC. The problem is that this client is
really popular. This is better to be kludged.

Signed-off-by: Pavel Butsykin <[email protected]>
Signed-off-by: Denis V. Lunev <[email protected]>
CC: Gerd Hoffmann <[email protected]>
CC: Paolo Bonzini <[email protected]>
Signed-off-by: Gerd Hoffmann <[email protected]>