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/ssd.h

@@ -6,6 +6,13 @@
 #include "common/edge.h"
 #include "config/types.h"
 
+enum ssd_active_state {
+	SSD_INACTIVE = 0,
+	SSD_ACTIVE = 1,
+};
+
+#define FOR_EACH_ACTIVE_STATE(active) for (active = SSD_INACTIVE; active <= SSD_ACTIVE; active++)
+
 struct wlr_cursor;
 
 /*
@@ -36,11 +43,8 @@ enum ssd_part_type {
 	LAB_SSD_BUTTON,
 
 	LAB_SSD_PART_TITLEBAR,
-	LAB_SSD_PART_TITLEBAR_CORNER_RIGHT,
-	LAB_SSD_PART_TITLEBAR_CORNER_LEFT,
 	LAB_SSD_PART_TITLE,
 
-	/* shared by shadows, borders and extents */
 	LAB_SSD_PART_CORNER_TOP_LEFT,
 	LAB_SSD_PART_CORNER_TOP_RIGHT,
 	LAB_SSD_PART_CORNER_BOTTOM_RIGHT,
@@ -64,8 +68,8 @@ enum ssd_part_type {
 
 /* Forward declare arguments */
 struct ssd;
-struct ssd_button;
 struct ssd_hover_state;
+struct ssd_part;
 struct view;
 struct wlr_scene;
 struct wlr_scene_node;
@@ -96,12 +100,17 @@ struct ssd_hover_state *ssd_hover_state_new(void);
 void ssd_update_button_hover(struct wlr_scene_node *node,
 	struct ssd_hover_state *hover_state);
 
-enum ssd_part_type ssd_button_get_type(const struct ssd_button *button);
-struct view *ssd_button_get_view(const struct ssd_button *button);
+enum ssd_part_type ssd_part_get_type(const struct ssd_part *part);
+struct view *ssd_part_get_view(const struct ssd_part *part);
 
 /* Public SSD helpers */
-enum ssd_part_type ssd_get_part_type(const struct ssd *ssd,
-	struct wlr_scene_node *node, struct wlr_cursor *cursor);
+
+/*
+ * Returns a part type that represents a mouse context like "Top", "Left" and
+ * "TRCorner" when the cursor is on the window border or resizing handle.
+ */
+enum ssd_part_type ssd_get_resizing_type(const struct ssd *ssd,
+	struct wlr_cursor *cursor);
 enum lab_edge ssd_resize_edges(enum ssd_part_type type);
 bool ssd_part_contains(enum ssd_part_type whole, enum ssd_part_type candidate);
 enum lab_ssd_mode ssd_mode_parse(const char *mode);