xwayland: Restrict `_XWAYLAND_ALLOW_COMMITS` to the X11 window/compositing manager
This is something that was asked by @pq in the original patch submission when adding support for _XWAYLAND_ALLOW_COMMITS
back in 2016.
https://lists.freedesktop.org/archives/xorg-devel/2016-December/051959.html
I expect the property to be written only from the XWM. Should I verify that somehow? Can I even identify the XWM here?
At the time, we didn't keep track of the X11 window manager client, so adding that check was a bit involved.
Since then, we identify the X11 window manager client id in Xwayland, so we can easily implement that feature.
So we can now restrict access to the _XWAYLAND_ALLOW_COMMITS property to read-only, except for the X11 window manager and the Xserver itself.
/cc @pq