x11 backend: Ignore expose if frame is pending

When resizing rootston with the mouse, the result is really slow. One
can see that rootston needs quite a while for drawing the newly visible
area. This is because every single expose event is handled on its own
and causes (apparently) a full repaint or at least a swap.

This commit improves things by only causing a new frame if none is
pending already.

With this change, there is almost no delay in rootston drawing the newly
visible area.

Signed-off-by: Uli Schlachter <psychon@znc.in>

backend/x11/backend.c

@@ -74,7 +74,7 @@ static void handle_x11_event(struct wlr_x11_backend *x11,
 		xcb_expose_event_t *ev = (xcb_expose_event_t *)event;
 		struct wlr_x11_output *output =
 			x11_output_from_window_id(x11, ev->window);
-		if (output != NULL) {
+		if (output != NULL && !output->wlr_output.frame_pending) {
 			wlr_output_send_frame(&output->wlr_output);
 		}
 		break;