ssd: unify struct ssd_part with struct node_descriptor

struct ssd_part and struct node_descriptor seem to have essentially the
same purpose: tag a wlr_scene_node with some extra data indicating what
we're using it for.

Also, as with enum ssd_part_type (now lab_node_type), ssd_part is used
for several types of nodes that are not part of SSD.

So instead of the current chaining (node_descriptor -> ssd_part), let's
flatten/unify the two structs.

In detail:

- First, merge node_descriptor_type into lab_node_type.
- Add a separate view pointer in node_descriptor, since in the case of
  SSD buttons we need separate view and button data pointers.
- Rename ssd_part_button to simply ssd_button. It no longer contains
  an ssd_part as base.
- Add node_try_ssd_button_from_node() which replaces
  node_ssd_part_from_node() + button_try_from_ssd_part().
- Factor out ssd_button_free() to be called in node descriptor destroy.
- Finally, get_cursor_context() needs a little reorganization to handle
  the unified structs.

Overall, this simplifies the code a bit, and in my opinion makes it
easier to understand. No functional change intended.

src/node.c

@@ -3,31 +3,29 @@
 #include <assert.h>
 #include <stdlib.h>
 #include "common/mem.h"
-
-static void
-descriptor_destroy(struct node_descriptor *node_descriptor)
-{
-	if (!node_descriptor) {
-		return;
-	}
-	wl_list_remove(&node_descriptor->destroy.link);
-	free(node_descriptor);
-}
+#include "ssd.h"
 
 static void
 handle_node_destroy(struct wl_listener *listener, void *data)
 {
 	struct node_descriptor *node_descriptor =
 		wl_container_of(listener, node_descriptor, destroy);
-	descriptor_destroy(node_descriptor);
+
+	if (node_type_contains(LAB_NODE_BUTTON, node_descriptor->type)) {
+		ssd_button_free(node_descriptor->data);
+	}
+
+	wl_list_remove(&node_descriptor->destroy.link);
+	free(node_descriptor);
 }
 
 void
 node_descriptor_create(struct wlr_scene_node *scene_node,
-		enum node_descriptor_type type, void *data)
+		enum lab_node_type type, struct view *view, void *data)
 {
 	struct node_descriptor *node_descriptor = znew(*node_descriptor);
 	node_descriptor->type = type;
+	node_descriptor->view = view;
 	node_descriptor->data = data;
 	node_descriptor->destroy.notify = handle_node_destroy;
 	wl_signal_add(&scene_node->events.destroy, &node_descriptor->destroy);
@@ -39,9 +37,7 @@ node_view_from_node(struct wlr_scene_node *wlr_scene_node)
 {
 	assert(wlr_scene_node->data);
 	struct node_descriptor *node_descriptor = wlr_scene_node->data;
-	assert(node_descriptor->type == LAB_NODE_DESC_VIEW
-		|| node_descriptor->type == LAB_NODE_DESC_XDG_POPUP);
-	return (struct view *)node_descriptor->data;
+	return node_descriptor->view;
 }
 
 struct lab_layer_surface *
@@ -49,7 +45,7 @@ node_layer_surface_from_node(struct wlr_scene_node *wlr_scene_node)
 {
 	assert(wlr_scene_node->data);
 	struct node_descriptor *node_descriptor = wlr_scene_node->data;
-	assert(node_descriptor->type == LAB_NODE_DESC_LAYER_SURFACE);
+	assert(node_descriptor->type == LAB_NODE_LAYER_SURFACE);
 	return (struct lab_layer_surface *)node_descriptor->data;
 }
 
@@ -58,7 +54,7 @@ node_layer_popup_from_node(struct wlr_scene_node *wlr_scene_node)
 {
 	assert(wlr_scene_node->data);
 	struct node_descriptor *node_descriptor = wlr_scene_node->data;
-	assert(node_descriptor->type == LAB_NODE_DESC_LAYER_POPUP);
+	assert(node_descriptor->type == LAB_NODE_LAYER_POPUP);
 	return (struct lab_layer_popup *)node_descriptor->data;
 }
 
@@ -67,24 +63,28 @@ node_menuitem_from_node(struct wlr_scene_node *wlr_scene_node)
 {
 	assert(wlr_scene_node->data);
 	struct node_descriptor *node_descriptor = wlr_scene_node->data;
-	assert(node_descriptor->type == LAB_NODE_DESC_MENUITEM);
+	assert(node_descriptor->type == LAB_NODE_MENUITEM);
 	return (struct menuitem *)node_descriptor->data;
 }
 
-struct ssd_part *
-node_ssd_part_from_node(struct wlr_scene_node *wlr_scene_node)
-{
-	assert(wlr_scene_node->data);
-	struct node_descriptor *node_descriptor = wlr_scene_node->data;
-	assert(node_descriptor->type == LAB_NODE_DESC_SSD_PART);
-	return (struct ssd_part *)node_descriptor->data;
-}
-
 struct scaled_buffer *
 node_scaled_buffer_from_node(struct wlr_scene_node *wlr_scene_node)
 {
 	assert(wlr_scene_node->data);
 	struct node_descriptor *node_descriptor = wlr_scene_node->data;
-	assert(node_descriptor->type == LAB_NODE_DESC_SCALED_BUFFER);
+	assert(node_descriptor->type == LAB_NODE_SCALED_BUFFER);
 	return (struct scaled_buffer *)node_descriptor->data;
 }
+
+struct ssd_button *
+node_try_ssd_button_from_node(struct wlr_scene_node *wlr_scene_node)
+{
+	assert(wlr_scene_node->data);
+	struct node_descriptor *node_descriptor = wlr_scene_node->data;
+
+	if (node_type_contains(LAB_NODE_BUTTON, node_descriptor->type)) {
+		return (struct ssd_button *)node_descriptor->data;
+	}
+
+	return NULL;
+}