drm: Cancel any scheduled repaints in the leave vt handler

If we don't cancel the repaint, we end up pointlessly redrawing the output.
What's worse is that pageflipping to the new buffer eventually fails and
we miss the finish_frame callback, leaving the compositor stuck when we
re-enter the vt.

src/compositor-drm.c

@@ -835,10 +835,21 @@ vt_func(struct weston_compositor *compositor, int event)
 		ec->prev_state = compositor->state;
 		compositor->state = WESTON_COMPOSITOR_SLEEPING;
 
+		/* If we have a repaint scheduled (either from a
+		 * pending pageflip or the idle handler), make sure we
+		 * cancel that so we don't try to pageflip when we're
+		 * vt switched away.  The SLEEPING state will prevent
+		 * further attemps at repainting.  When we switch
+		 * back, we schedule a repaint, which will process
+		 * pending frame callbacks. */
+
+		wl_list_for_each(output, &ec->base.output_list, link) {
+			output->repaint_needed = 0;
+			drm_output_set_cursor(output, NULL);
+		}
+
 		wl_list_for_each(input, &compositor->input_device_list, link)
 			evdev_remove_devices(input);
-		wl_list_for_each(output, &ec->base.output_list, link)
-			drm_output_set_cursor(output, NULL);
 
 		if (drmDropMaster(ec->drm.fd) < 0)
 			fprintf(stderr, "failed to drop master: %m\n");

src/compositor.c

@@ -840,9 +840,8 @@ struct weston_frame_callback {
 };
 
 static void
-repaint(void *data, int msecs)
+repaint(struct weston_output *output, int msecs)
 {
-	struct weston_output *output = data;
 	struct weston_compositor *compositor = output->compositor;
 	struct weston_animation *animation, *next;
 	struct weston_frame_callback *cb, *cnext;
@@ -865,7 +864,11 @@ repaint(void *data, int msecs)
 static void
 idle_repaint(void *data)
 {
-	repaint(data, weston_compositor_get_time());
+	struct weston_output *output = data;
+
+	/* An idle repaint may have been cancelled by vt switching away. */
+	if (output->repaint_needed)
+		repaint(output, weston_compositor_get_time());
 }
 
 WL_EXPORT void