compositor-x11: add assert to avoid misuse of x11_backend_deliver_button_event

The x11_backend_deliver_button_event can be called with any
xcb_generic_event. The assert check if the call is done with the
expected events.

Signed-off-by: Benoit Gschwind <gschwind@gnu-log.net>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>

libweston/compositor-x11.c

@@ -1068,6 +1068,9 @@ x11_backend_deliver_button_event(struct x11_backend *b,
 	struct x11_output *output;
 	struct weston_pointer_axis_event weston_event;
 
+	assert(event->response_type == XCB_BUTTON_PRESS ||
+	       event->response_type == XCB_BUTTON_RELEASE);
+
 	output = x11_backend_find_output(b, button_event->event);
 	if (!output)
 		return;