Michel Dänzer authored Jun 19, 2020 and Olivier Fourdan committed Jul 20, 2020

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.