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.
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