Huge performance increase of getCanvas

Using the imageData.data.set() method you can use the webGL pixeldata directly, avoiding the need to loop through the pixels.  Tested and confirmed to be working on the latest versions of Chrome/Firefox/IE.

The speed up is enormous.  Turning a 1920x1080 DisplayObjectContainer into a 2d canvas, takes only 60ms, compared with 300ms+ using the old method.