Add scaled_rect_buffer

scaled_rect_buffer is an implementation of scaled_scene_buffer and shows
an auto-scaling bordered rectangle. This is intended for menu borders,
but can be also useful for other elements like window switcher items.

We will support rounded corners for scaled_rect_buffer in the future.

src/common/meson.build

@@ -14,6 +14,7 @@ labwc_sources += files(
   'parse-bool.c',
   'parse-double.c',
   'scaled-font-buffer.c',
+  'scaled-rect-buffer.c',
   'scaled-scene-buffer.c',
   'scene-helpers.c',
   'set.c',