CanvasRenderer: Account for spirite rotation when computing clear area.

... plus other minor fixes.

The approach I took was conservative. A smaller clear box could be computed based on the actual rotation, but it would involve sines and cosines.

The problem was particularly visible when zooming in and out.

![canvasrenderer-sprite-zoom](https://f.cloud.github.com/assets/1000017/1609348/5bc5a3ea-553e-11e3-826f-1185e5258216.png)