wlr_scene: Immediately apply pending output commit damage

There were two problems with the old implementation:
1. wlr_scene_output_commit would bail early if a frame wasn't requested
and there was no commit damage, however commit damage could never accumulate
until rendering happens. The check was subtly wrong as a result.
2. Previously, we would fill the pending commit damage based on the
current state of the damage ring. However, during direct scanout, the
damage would accumulate which would mean we would submit damage from
previous frames even if we didn't need to.

types/scene/wlr_scene.c

@@ -332,6 +332,27 @@ static void scene_output_damage(struct wlr_scene_output *scene_output,
 		const pixman_region32_t *region) {
 	if (wlr_damage_ring_add(&scene_output->damage_ring, region)) {
 		wlr_output_schedule_frame(scene_output->output);
+
+		struct wlr_output *output = scene_output->output;
+		enum wl_output_transform transform =
+			wlr_output_transform_invert(scene_output->output->transform);
+
+		int width = output->width;
+		int height = output->height;
+		if (transform & WL_OUTPUT_TRANSFORM_90) {
+			width = output->height;
+			height = output->width;
+		}
+
+		pixman_region32_t frame_damage;
+		pixman_region32_init(&frame_damage);
+		wlr_region_transform(&frame_damage, region, transform, width, height);
+
+		pixman_region32_union(&scene_output->pending_commit_damage,
+			&scene_output->pending_commit_damage, &frame_damage);
+		pixman_region32_intersect_rect(&scene_output->pending_commit_damage,
+			&scene_output->pending_commit_damage, 0, 0, output->width, output->height);
+		pixman_region32_fini(&frame_damage);
 	}
 }
 
@@ -1654,14 +1675,6 @@ static void output_state_apply_damage(const struct render_data *data,
 		struct wlr_output_state *state) {
 	struct wlr_scene_output *output = data->output;
 
-	pixman_region32_t frame_damage;
-	pixman_region32_init(&frame_damage);
-	pixman_region32_copy(&frame_damage, &output->damage_ring.current);
-	transform_output_damage(&frame_damage, data);
-	pixman_region32_union(&output->pending_commit_damage,
-		&output->pending_commit_damage, &frame_damage);
-	pixman_region32_fini(&frame_damage);
-
 	wlr_output_state_set_damage(state, &output->pending_commit_damage);
 }