xwm: Use Xwayland allow commits for repaint

Initially, `_XWAYLAND_ALLOW_COMMITS` was introduced in commit 7ace831c
to avoid drawing the window content before it's ready to be shown.

But a repaint might also be triggered by the client damages before the
XWM has finished drawing its window decorations and drop shadows, which
previously was not too much of an issue since the XWM could still
finish updating the X11 window after the buffer was submitted.

However, with the addition of multiple window buffers in Xwayland [1]
which are aimed at preventing the X11 clients from updating the buffer
after it's been committed, this is no longer possible.

As a result, the use of multiple window buffers in Xwayland can cause
ugly repainting effects of the decorations if the buffer is submitted
before the XWM has finished painting its decorations.

Use the X11 property `_XWAYLAND_ALLOW_COMMITS` can be used to avoid
this, by controlling when Xwayland should commit changes to the Wayland
surface.

[1] xorg/xserver!316
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
3 jobs for extend-allow-commits in 2 minutes and 12 seconds (queued for 1 minute and 34 seconds)
Status Job ID Name Coverage
  Container Prep
passed #995725
container_prep

00:00:38

 
  Build
passed #995726
build-native-meson-default-options

00:01:02

passed #995727
build-native-meson-no-gl-renderer

00:00:30