Added a method of optionally clipping wlr_scene_tree children

The two primary use-cases that I have in mind:
- Ensuring that layer surfaces don't spill over to adjacent outputs
- Clipping workspace translation animations to a single output

include/wlr/types/wlr_scene.h

@@ -92,6 +92,11 @@ struct wlr_scene_tree {
 	struct wlr_scene_node node;
 
 	struct wl_list children; // wlr_scene_node.link
+
+	struct {
+		// The clipping region in the tree nodes-local coordinate space
+		struct wlr_box clip;
+	} WLR_PRIVATE;
 };
 
 /** The root scene-graph node. */
@@ -386,6 +391,17 @@ void wlr_scene_set_color_manager_v1(struct wlr_scene *scene, struct wlr_color_ma
  */
 struct wlr_scene_tree *wlr_scene_tree_create(struct wlr_scene_tree *parent);
 
+/**
+ * Sets a cropping region for any nodes that are children of this scene tree.
+ * Note that clip boxes cascade down the tree, as in the clip boxes are
+ * intersected with each other from the root to the leaves.
+ * The clip coordinate space will be that of the tree node.
+ *
+ * A NULL or empty clip will disable clipping.
+ */
+void wlr_scene_tree_set_clip(struct wlr_scene_tree *tree,
+		const struct wlr_box *clip);
+
 /**
  * Add a node displaying a single surface to the scene-graph.
  *