Added a method to Texture to get the bounds of an area matching a color

Behaves similarly to BitmapData#getColorBoundsRect
(http://help.adobe.com/en_US/as2/reference/flashlite/WS5F7E996C-A0F6-4d89-BC17-69622D209F21.html).

For stage3d this uses `getColorBoundsRect`, for HTML5 it will iterate over the raw pixel values.