patch 9.2.0733: GTK3: GUI slow on X11 since dropping the alpha channel

patch 9.2.0733: GTK3: GUI slow on X11 since dropping the alpha channel

Problem:  With the GTK3 GUI, scrolling and moving the window are slow on
          X11 on setups where the accelerated drawing path expects an
          ARGB surface (after v9.2.0646)
Solution: Choose the offscreen surface content per display backend: keep the
          alpha-less surface on Wayland, where it avoids needless alpha
          compositing, and use an ARGB surface on X11 again, which is the
          accelerated path there, restoring the speed (Hirohito Higashi).

fixes:  #20603
closes: #20634

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>