xwayland: Clear tablet cursor pending frame cb

If the tablet tool is moved out of proximity before the cursor's pending
frame callback is received, any further attempts to update the cursor
will fail because the frame callback is still pending.

Make sure to clear any cursor pending frame when the tool gets in
proximity again, similar to what we do when the pointer re-enters a
surface, so that the cursor updates aren't discarded.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
See-also: https://gitlab.gnome.org/GNOME/mutter/-/issues/1969
Reviewed-by: Carlos Garnacho <carlosg@gnome.org>
(cherry picked from commit 35c5664f)
3 jobs for backport-wacom-cursor-fix in 1 minute and 37 seconds (queued for 3 seconds)
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  Docker Image
passed debian-buster #14946249

00:00:14

 
  Build And Test
passed meson #14946250

00:01:23

passed meson-noglamor #14946251

00:01:21