xwayland: Hold a pixmap reference in struct xwl_present_event

In the log of the commit below, I claimed this wasn't necessary on the
1.20 branch, but this turned out to be wrong: It meant that
event->buffer could already be destroyed in xwl_present_free_event,
resulting in use-after-free and likely a crash.

Fixes: 22c0808a "xwayland: Free all remaining events in
3 jobs for xwl_present_event-pixmap-reference in 3 minutes and 48 seconds (queued for 2 seconds)