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.