Implemented IOSCanvas.transform, deprecated Canvas.setTransform.

The deprecation was for two reasons:

1. Setting the transform outright wipes out any preconfigured transform that
PlayN puts on the canvas to make things work correctly in HiDPI mode. So that's
bad.

2. It's not possible to implement Canvas.setTransform on iOS. You can't set the
transform outright, you can only append to it.