Unit Testing, Tweaks
#ADD: Added ContentProvider.DisposeDefaultContent method for system shutdown.
#ADD: Added unit tests for all shapes and transformation operations to CanvasTest.
#ADD: Added Scene.SwitchTo method which allows safely issuing Scene switches regardless of calling context.
#ADD: Added unit tests for Scene.SwitchTo behavior.
#CHANGE: Scene.Current setter is now private. Use Scene.SwitchTo instead.
#CHANGE: Canvas internal dash textures are now properly registered using the ContentProvider, so they remain functional after system reset.
#CHANGE: Texture.Checkerboard no longer uses downscale filtering.
#FIX: Fixed Duality unit testing environment not properly shutting down.