ssd: rework titlebar button rendering

- fix that icons for normal/hovered/rounded buttons are not placed
  exactly the same position
- fix blurry window button icons in scaled outputs

This commit introduces lab_img and scaled_img_buffer and uses them for
rendering icons in the window titlebar. Now the process of rendering
button icons are split into 2 phases: loading with lab_img_load() and
creating scene-nodes for them with scaled_img_buffer_create(). This
might incur some additional overhead since we no longer preload icon
textures, but the rendering of icon only happens for the first window
as backing buffers are shared and the overhead won't be noticeable.
This commit also simplifies the process of centering icon buffer in the
button, by creating icon buffers in a fixed geometry via
lab_img_render().

include/common/box.h

@@ -13,14 +13,13 @@ void box_union(struct wlr_box *box_dest, struct wlr_box *box_a,
 	struct wlr_box *box_b);
 
 /*
- * Fits and centers a content box (width & height) within a bounding box
- * (max_width & max_height). The content box is downscaled if necessary
- * (preserving aspect ratio) but not upscaled.
+ * Fits and centers a content box (width & height) within a bounding box.
+ * The content box is downscaled if necessary (preserving aspect ratio) but
+ * not upscaled.
  *
  * The returned x & y coordinates are the centered content position
  * relative to the top-left corner of the bounding box.
  */
-struct wlr_box box_fit_within(int width, int height, int max_width,
-	int max_height);
+struct wlr_box box_fit_within(int width, int height, struct wlr_box *bounding_box);
 
 #endif /* LABWC_BOX_H */