the display adapter code. When ready, the common gui code should handle it
completely. The snapshot handler should set a flag 'snapshot_mode' and then it
should force a display update. The graphics update code should be redirected to
some code that 'draws' to a snapshot buffer. These first steps are done:
- store some guest display settings in the gui code (text / graphics switch,
x / y resolution and bpp)
- split 8 bpp palette update code in a common and a specific part and store the
palette values in the gui code
