examples/scene-graph: demonstrate scene_rect node type
examples/scene-graph: demonstrate scene_rect node type

Add RECT nodes to the scene-graph demo to illustrate how they are used.
Here, we add a solid rectangle behind each surface as a quick-and-dirty
border, handling surface.commit in order to size it appropriately.

swaywm/wlroots,swaywm/wlroots,SirCmpwn/wlroots