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`.

src/ssd/ssd-extents.c

@@ -11,22 +11,11 @@
 #include "theme.h"
 #include "view.h"
 
-static struct ssd_part *
-add_extent(struct wl_list *part_list, enum ssd_part_type type,
-		struct wlr_scene_tree *parent)
-{
-	float invisible[4] = { 0.0f, 0.0f, 0.0f, 0.0f };
-	struct ssd_part *part = add_scene_part(part_list, type);
-	part->node = &wlr_scene_rect_create(parent, 0, 0, invisible)->node;
-	return part;
-}
-
 void
 ssd_extents_create(struct ssd *ssd)
 {
 	struct view *view = ssd->view;
 	struct theme *theme = view->server->theme;
-	struct wl_list *part_list = &ssd->extents.parts;
 
 	int border_width = MAX(0, MAX(rc.resize_minimum_area, theme->border_width));
 
@@ -35,14 +24,14 @@ ssd_extents_create(struct ssd *ssd)
 	if (view->fullscreen || view->maximized == VIEW_AXIS_BOTH) {
 		wlr_scene_node_set_enabled(&parent->node, false);
 	}
-	wl_list_init(&ssd->extents.parts);
 	wlr_scene_node_set_position(&parent->node,
 		-border_width, -(ssd->titlebar.height + border_width));
 
-	add_extent(part_list, LAB_SSD_PART_TOP, parent);
-	add_extent(part_list, LAB_SSD_PART_LEFT, parent);
-	add_extent(part_list, LAB_SSD_PART_RIGHT, parent);
-	add_extent(part_list, LAB_SSD_PART_BOTTOM, parent);
+	float invisible[4] = { 0.0f, 0.0f, 0.0f, 0.0f };
+	ssd->extents.top = wlr_scene_rect_create(parent, 0, 0, invisible);
+	ssd->extents.left = wlr_scene_rect_create(parent, 0, 0, invisible);
+	ssd->extents.right = wlr_scene_rect_create(parent, 0, 0, invisible);
+	ssd->extents.bottom = wlr_scene_rect_create(parent, 0, 0, invisible);
 
 	/* Initial manual update to keep X11 applications happy */
 	ssd_extents_update(ssd);
@@ -121,10 +110,6 @@ ssd_extents_update(struct ssd *ssd)
 	int border_width = MAX(rc.resize_minimum_area, theme->border_width);
 	int extended_area = MAX(0, rc.resize_minimum_area - theme->border_width);
 
-	struct ssd_part *part;
-	struct wlr_scene_rect *rect;
-	struct wlr_box target;
-
 	/* Make sure we update the y offset based on titlebar shown / hidden */
 	wlr_scene_node_set_position(&ssd->extents.tree->node,
 		-border_width, -(ssd->titlebar.height + border_width));
@@ -151,43 +136,19 @@ ssd_extents_update(struct ssd *ssd)
 	wlr_scene_node_coords(&ssd->extents.tree->node, &base_x, &base_y);
 	pixman_region32_translate(&usable, -base_x, -base_y);
 
-	wl_list_for_each(part, &ssd->extents.parts, link) {
-		rect = wlr_scene_rect_from_node(part->node);
-		switch (part->type) {
-		case LAB_SSD_PART_TOP:
-			target.x = 0;
-			target.y = 0;
-			target.width = full_width + extended_area * 2;
-			target.height = extended_area;
-			break;
-		case LAB_SSD_PART_LEFT:
-			target.x = 0;
-			target.y = extended_area;
-			target.width = extended_area;
-			target.height = full_height;
-			break;
-		case LAB_SSD_PART_RIGHT:
-			target.x = extended_area + full_width;
-			target.y = extended_area;
-			target.width = extended_area;
-			target.height = full_height;
-			break;
-		case LAB_SSD_PART_BOTTOM:
-			target.x = 0;
-			target.y = extended_area + full_height;
-			target.width = full_width + extended_area * 2;
-			target.height = extended_area;
-			break;
-		default:
-			/* not reached */
-			assert(false);
-			/* suppress warnings with NDEBUG */
-			target = (struct wlr_box){0};
-		}
+	resize_extent_within_usable(ssd->extents.top, &usable,
+		0, 0,
+		full_width + extended_area * 2, extended_area);
+	resize_extent_within_usable(ssd->extents.left, &usable,
+		0, extended_area,
+		extended_area, full_height);
+	resize_extent_within_usable(ssd->extents.right, &usable,
+		extended_area + full_width, extended_area,
+		extended_area, full_height);
+	resize_extent_within_usable(ssd->extents.bottom, &usable,
+		0, extended_area + full_height,
+		full_width + extended_area * 2, extended_area);
 
-		resize_extent_within_usable(rect, &usable,
-			target.x, target.y, target.width, target.height);
-	}
 	pixman_region32_fini(&usable);
 }
 
@@ -198,7 +159,6 @@ ssd_extents_destroy(struct ssd *ssd)
 		return;
 	}
 
-	ssd_destroy_parts(&ssd->extents.parts);
 	wlr_scene_node_destroy(&ssd->extents.tree->node);
-	ssd->extents.tree = NULL;
+	ssd->extents = (struct ssd_extents_scene){0};
 }