console: untangle gfx & txt updates

console: untangle gfx & txt updates

Stop abusing displaysurface fields for text mode displays.
(bpp = 0, width = cols, height = lines).

Add flags to displaystate indicating whenever text mode display
(curses) or gfx mode displays (sdl, vnc, ...) are present.

Add separate displaychangelistener callbacks for text / gfx mode
resize & updates.

This allows to enable gfx and txt diplays at the same time and also
paves the way for more cleanups in the future.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>