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>
5 jobs for xwayland-tablet-proximity-cursor-fix in 4 minutes and 53 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Docker Image
passed debian-buster #14936446

00:00:20

 
  Build And Test
passed autotools #14936447

00:04:33

passed meson #14936448

00:01:59

passed meson-noglamor #14936449

00:01:18

passed mingw-cross-build #14936450

00:00:36