scaled-scene-buffer: restructure source files

- Rename `scaled_scene_buffer` to `scaled_buffer`. This makes it clear
  that `scaled_{font,img,icon}_buffers` are implementations of it.
- Move the files from `src/common` to `src/scaled-buffer` as
  `scaled_icon_buffer` heavily depends on `server` and `view` etc.

src/desktop.c

@@ -350,7 +350,7 @@ get_cursor_context(struct server *server)
 				return ret;
 			case LAB_NODE_DESC_NODE:
 			case LAB_NODE_DESC_TREE:
-			case LAB_NODE_DESC_SCALED_SCENE_BUFFER:
+			case LAB_NODE_DESC_SCALED_BUFFER:
 				break;
 			}
 		}