scene: replace surface_at() with node_at()

With the addition of a non-surface node type, it was unclear how such
nodes should interact with scene_node_surface_at().  For example, if the
topmost node at the given point is a RECT, should the function treat
that node as transparent and continue searching, or as opaque and return
(probably) NULL?

Instead, replace the function with one returning a scene_node, which
will allow for more consistent behavior across different node types.
Compositors can downcast scene_surface nodes via the now-public
wlr_scene_surface_from_node() if they need access to the surface itself.

include/wlr/types/wlr_scene.h

@@ -114,12 +114,13 @@ void wlr_scene_node_reparent(struct wlr_scene_node *node,
 void wlr_scene_node_for_each_surface(struct wlr_scene_node *node,
 	wlr_surface_iterator_func_t iterator, void *user_data);
 /**
- * Find a surface in this scene-graph that accepts input events at the given
- * layout-local coordinates. Returns the surface and coordinates relative to
- * the returned surface, or NULL if no surface is found at that location.
+ * Find the topmost node in this scene-graph that contains the point at the
+ * given layout-local coordinates. (For surface nodes, this means accepting
+ * input events at that point.) Returns the node and coordinates relative to the
+ * returned node, or NULL if no node is found at that location.
  */
-struct wlr_surface *wlr_scene_node_surface_at(struct wlr_scene_node *node,
-	double lx, double ly, double *sx, double *sy);
+struct wlr_scene_node *wlr_scene_node_at(struct wlr_scene_node *node,
+	double lx, double ly, double *nx, double *ny);
 
 /**
  * Create a new scene-graph.
@@ -142,6 +143,8 @@ void wlr_scene_render_output(struct wlr_scene *scene, struct wlr_output *output,
 struct wlr_scene_surface *wlr_scene_surface_create(struct wlr_scene_node *parent,
 	struct wlr_surface *surface);
 
+struct wlr_scene_surface *wlr_scene_surface_from_node(struct wlr_scene_node *node);
+
 /**
  * Add a node displaying a solid-colored rectangle to the scene-graph.
  */

types/wlr_scene.c

@@ -13,7 +13,7 @@ static struct wlr_scene *scene_root_from_node(struct wlr_scene_node *node) {
 	return (struct wlr_scene *)node;
 }
 
-static struct wlr_scene_surface *scene_surface_from_node(
+struct wlr_scene_surface *wlr_scene_surface_from_node(
 		struct wlr_scene_node *node) {
 	assert(node->type == WLR_SCENE_NODE_SURFACE);
 	return (struct wlr_scene_surface *)node;
@@ -74,7 +74,7 @@ void wlr_scene_node_destroy(struct wlr_scene_node *node) {
 		free(scene);
 		break;
 	case WLR_SCENE_NODE_SURFACE:;
-		struct wlr_scene_surface *scene_surface = scene_surface_from_node(node);
+		struct wlr_scene_surface *scene_surface = wlr_scene_surface_from_node(node);
 		wl_list_remove(&scene_surface->surface_destroy.link);
 		free(scene_surface);
 		break;
@@ -198,7 +198,7 @@ static void scene_node_for_each_surface(struct wlr_scene_node *node,
 	ly += node->state.y;
 
 	if (node->type == WLR_SCENE_NODE_SURFACE) {
-		struct wlr_scene_surface *scene_surface = scene_surface_from_node(node);
+		struct wlr_scene_surface *scene_surface = wlr_scene_surface_from_node(node);
 		user_iterator(scene_surface->surface, lx, ly, user_data);
 	}
 
@@ -213,8 +213,8 @@ void wlr_scene_node_for_each_surface(struct wlr_scene_node *node,
 	scene_node_for_each_surface(node, 0, 0, user_iterator, user_data);
 }
 
-struct wlr_surface *wlr_scene_node_surface_at(struct wlr_scene_node *node,
-		double lx, double ly, double *sx, double *sy) {
+struct wlr_scene_node *wlr_scene_node_at(struct wlr_scene_node *node,
+		double lx, double ly, double *nx, double *ny) {
 	if (!node->state.enabled) {
 		return NULL;
 	}
@@ -225,24 +225,40 @@ struct wlr_surface *wlr_scene_node_surface_at(struct wlr_scene_node *node,
 
 	struct wlr_scene_node *child;
 	wl_list_for_each_reverse(child, &node->state.children, state.link) {
-		struct wlr_surface *surface =
-			wlr_scene_node_surface_at(child, lx, ly, sx, sy);
-		if (surface != NULL) {
-			return surface;
+		struct wlr_scene_node *node =
+			wlr_scene_node_at(child, lx, ly, nx, ny);
+		if (node != NULL) {
+			return node;
 		}
 	}
 
-	if (node->type == WLR_SCENE_NODE_SURFACE) {
-		struct wlr_scene_surface *scene_surface = scene_surface_from_node(node);
+	switch (node->type) {
+	case WLR_SCENE_NODE_SURFACE:;
+		struct wlr_scene_surface *scene_surface = wlr_scene_surface_from_node(node);
 		if (wlr_surface_point_accepts_input(scene_surface->surface, lx, ly)) {
-			if (sx != NULL) {
-				*sx = lx;
+			if (nx != NULL) {
+				*nx = lx;
+			}
+			if (ny != NULL) {
+				*ny = ly;
+			}
+			return &scene_surface->node;
 		}
-			if (sy != NULL) {
-				*sy = ly;
+		break;
+	case WLR_SCENE_NODE_RECT:;
+		struct wlr_scene_rect *rect = scene_rect_from_node(node);
+		if (lx >= 0 && lx < rect->width && ly >= 0 && ly < rect->height) {
+			if (nx != NULL) {
+				*nx = lx;
+			}
+			if (ny != NULL) {
+				*ny = ly;
 			}
-			return scene_surface->surface;
+			return &rect->node;
 		}
+		break;
+	default:
+		break;
 	}
 
 	return NULL;
@@ -342,7 +358,7 @@ static void render_node_iterator(struct wlr_scene_node *node,
 		/* Root node has nothing to render itself */
 		break;
 	case WLR_SCENE_NODE_SURFACE:;
-		struct wlr_scene_surface *scene_surface = scene_surface_from_node(node);
+		struct wlr_scene_surface *scene_surface = wlr_scene_surface_from_node(node);
 		struct wlr_surface *surface = scene_surface->surface;
 
 		struct wlr_texture *texture = wlr_surface_get_texture(surface);