Add debug rectangle size rendering, auto-resize containers to fit all children.

TestGame/TestGame.cs

@@ -20,6 +20,7 @@ public class TestGame : Game
         InitializeSystemsDefault();
 
         _uiSystem = new UISystem(new ResourceRef<Style>(Guid.Empty));
+        _uiSystem.RenderDebugRects = true;
 
         _particleSystem = new ParticleSystem();