Add a means of exposing the underlying Bitmap

[ZacSweers]

Right now, the library doesn't appear to make retrieving the underlying bitmap in a Drawee. While this is understandable, it also makes postprocessing impossible for any users that want to do it.

An example would be for use in the Palette library, which requires an input Bitmap for the color extraction.

[tyronen]

Fresco has a postprocessor facility which sounds like what you are looking for.