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 xwl_present_cleanup"