It may track a non-toplevel window which fully covers the area of the
window pixmap / Wayland surface. It is now used instead of
xwl_window::toplevel for updating the Wayland surface contents.
The surface_window can now hit the Present page flip path while it's
automatically redirected.
v2:
* Use "surface_window" instead of "surf_win". (Olivier Fourdan)
* Add comment describing surface_window, and describe what
surface_window/toplevel are useful for respectively. (Olivier Fourdan)
* Use surface_window in xwl_realize_window.
v3:
* Backtrack up to the closest opaque ancestor in
xwl_window_update_surface_window. (Olivier Fourdan)
v4:
* Clean up logic for determining the surface window in
xwl_window_update_surface_window, and document it better.
* Handle window_get_damage(xwl_window->surface_window) returning NULL
in xwl_window_update_surface_window.
* Call xwl_window_update_surface_window after xwl_window_buffers_init
in ensure_surface_for_window, since the former may call
xwl_window_buffers_dispose.
* Rename surf/win_pix to surface/window_pixmap in
xwl_window_update_surface_window.
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1300>
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