Sprite nodes use Point instead of Vector2
Updates sprite test app
Refactors accounting for pivot points in sprites
Adds abstract NodeWithSize class in Sprites.
Refactors SpriteNode to Sprite
Refactors TransformNode to Node (may need to find another name as it conflicts with Sky's Node).
Sprite system now uses and caches transformation matrices.
R=abarth@chromium.org
Review URL: https://codereview.chromium.org/1180703002.
