ssd: clean up scene management

Our codebase for ssd scenes has grown with a lot of technical debts:
- We needed to call `ssd_get_part()` everywhere to get the scene node of a
  ssd part. We then needed to cast it to `wlr_scene_rect` and
  `wlr_scene_buffer`. This bloated our codebase and even blocked
  duplicated button types in `<titlebar><layout>`.
- `ssd_get_part_type()` was a dirty hack. It compared parent, grandparent
  and grandgrandparent of a node with each subtree in the ssd to get the
  part type of the node.

To resolve this issues, this commit changes how ssd scenes are managed:
- Access scene rects and scene buffers just as a member of `struct ssd`.
- `ssd_part` is now a attachment to a scene node that can be accessed via
  node_descriptor->data, with a new node-descriptor type
  `LAB_NODE_DESC_SSD_PART`. `LAB_NODE_DESC_SSD_BUTTON` is unified into it.

Now the scene graph under ssd->tree looks like below. The parentheses
indicate the type of ssd_part attached to the node:

ssd->tree (LAB_SSD_NONE)
+--titlebar (LAB_SSD_PART_TITLEBAR)
|  +--inactive
|  |  +--background bar
|  |  +--left corner
|  |  +--right corner
|  |  +--title (LAB_SSD_PART_TITLE)
|  |  +--iconify button (LAB_SSD_BUTTON_ICONIFY)
|  |  |  +--normal close icon image
|  |  |  +--hovered close icon image
|  |  |  +--...
|  |  +--window icon (LAB_SSD_BUTTON_WINDOW_ICON)
|  |  |  +--window icon image
|  |  +--...
|  +--active
|     +--...
+--border
|  +--inactive
|  |  +--top
|  |  +--...
|  +--active
|     +--top
|     +--...
+--shadow
|  +--inactive
|  |  +--top
|  |  +--...
|  +--active
|     +--top
|     +--...
+--extents
   +--top
   +--...

When hovering on SSD, `get_cursor_context()` traverses this scene node
from the leaf. If it finds a `ssd_part` attached to the node, it returns
`ssd_part_type` that represents the resizing direction, button types or
`Title`/`Titlebar`.

include/node.h

@@ -7,7 +7,7 @@ struct view;
 struct lab_layer_surface;
 struct lab_layer_popup;
 struct menuitem;
-struct ssd_button;
+struct ssd_part;
 struct scaled_scene_buffer;
 
 enum node_descriptor_type {
@@ -21,7 +21,7 @@ enum node_descriptor_type {
 	LAB_NODE_DESC_MENUITEM,
 	LAB_NODE_DESC_TREE,
 	LAB_NODE_DESC_SCALED_SCENE_BUFFER,
-	LAB_NODE_DESC_SSD_BUTTON,
+	LAB_NODE_DESC_SSD_PART,
 };
 
 struct node_descriptor {
@@ -44,7 +44,7 @@ struct node_descriptor {
  *   - LAB_NODE_DESC_LAYER_SURFACE  struct lab_layer_surface
  *   - LAB_NODE_DESC_LAYER_POPUP    struct lab_layer_popup
  *   - LAB_NODE_DESC_MENUITEM       struct menuitem
- *   - LAB_NODE_DESC_SSD_BUTTON     struct ssd_button
+ *   - LAB_NODE_DESC_SSD_PART       struct ssd_part
  */
 void node_descriptor_create(struct wlr_scene_node *scene_node,
 	enum node_descriptor_type type, void *data);
@@ -77,10 +77,10 @@ struct menuitem *node_menuitem_from_node(
 	struct wlr_scene_node *wlr_scene_node);
 
 /**
- * node_ssd_button_from_node - return ssd_button struct from node
+ * node_ssd_part_from_node - return ssd_part struct from node
  * @wlr_scene_node: wlr_scene_node from which to return data
  */
-struct ssd_button *node_ssd_button_from_node(
+struct ssd_part *node_ssd_part_from_node(
 	struct wlr_scene_node *wlr_scene_node);
 
 /**