added support for textures without premultiplied alpha

The current version of the runtime has issues with ATF textures. Different to the conventional Bitmap-based textures available in Stage3D, those textures do not use premultiplied alpha. This needs to be taken into account on rendering.

I tested this with a sample from the Starling user who reported this, and it took care of the problem. As you can see, the changes are really minimal. If you've got any questions about the change, or want me to do anything differently, just let me know.

Cheers!