This is needed for compositors that want to reserve space for
XWayland panels. Such a feature can be useful in a "transitional"
setup, where only the X11 window manager and compositor is replaced
but other components of an X11 desktop environment are still used.
This change simply reads the X11 property; the compositor is free
to ignore it. Thus, compositors that don't want to support such a
"transitional" feature are not impacted.
v2: Update xwayland_surface_associate()
John Lindgren
068280201a