StageGL doesn't implement `toDataURL`

There is no override on the `toDataURL` on StageGL. 

### Issue Details

* The regular toDataURL on Stage uses 2d context to draw a background. This functionality should be implemented in StageGL.
* WebGL canvas element returns a flipped image when called directly. @DavidHGillen suggested a 2d canvas may be needed as an interim step.

EaselJS/StageGL Version 1.0
