Default to cross fade animation for drawables.
More documentation updates
Make GifRequestBuilder return GifDrawables.
Add documentation for GlideAnimationFactory.
Add constant state to GifDrawable.
Avoid unnecessary get() calls.
Provide final frame dimensions to GifDrawable.

This allows the intrinsic width and height to be
constant which plays more nicely with the android
framework.
Avoid resource recycle loops.
Fix a gif recycling error.
Use frame dimensions to estimate gif memory size.