xwayland: Free all remaining events in xwl_present_cleanup

At the end of xwl_present_cleanup, these events aren't reachable
anymore, so if we don't free them first, they're leaked.

(cherry picked from commit 64565ea344fef0171497952ef75f019cb420fe3b)

v2:
* Simpler backport, no need to keep a reference to the pixmap on the
  1.20 branch.
3 jobs for server-1.20-branch in 6 minutes and 4 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #3690920
debian-testing

00:00:51

 
  Build And Test
passed #3690921
autotools-build-and-test

00:05:12

passed #3690922
meson-build-and-test

00:00:51