xwm: Use Xwayland allow commits for repaint
_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  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