wlr_scene: Introduce wlr_scene_output_needs_frame

It seems that some scene compositors want to avoid wlr_scene_output_commit
and use the lower lever wlr_scene_output_build_state. However, build
state does not early return if a frame is not needed so compositors will
implement the check themselves. Let's add a helper function that compositors
can use to implement the check.

Technically pending_commit_damage is a private interface, so this lets
compositors not interface with private interfaces to implement the check.

include/wlr/types/wlr_scene.h

@@ -529,6 +529,12 @@ struct wlr_scene_output_state_options {
 	struct wlr_swapchain *swapchain;
 };
 
+/**
+ * Returns true if scene wants to render a new frame. False, if no new frame
+ * is needed and an output commit can be skipped for the current frame.
+ */
+bool wlr_scene_output_needs_frame(struct wlr_scene_output *scene_output);
+
 /**
  * Render and commit an output.
  */

types/scene/wlr_scene.c

@@ -1818,10 +1818,14 @@ static bool scene_entry_try_direct_scanout(struct render_list_entry *entry,
 	return true;
 }
 
+bool wlr_scene_output_needs_frame(struct wlr_scene_output *scene_output) {
+	return scene_output->output->needs_frame || pixman_region32_not_empty(
+		&scene_output->pending_commit_damage);
+}
+
 bool wlr_scene_output_commit(struct wlr_scene_output *scene_output,
 		const struct wlr_scene_output_state_options *options) {
-	if (!scene_output->output->needs_frame && !pixman_region32_not_empty(
-			&scene_output->pending_commit_damage)) {
+	if (!wlr_scene_output_needs_frame(scene_output)) {
 		return true;
 	}