In xwl_glamor_eglstream_get_wl_buffer_for_pixmap. This can likely be hit
now with an SHM pixmap via the Present flip path. There might be other
corner cases.
Fixes: f3eb1684fa "xwayland: enable MIT-SHM shared pixmaps"
Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>