public void renderDeltas(int frameNumber, Bitmap baseBitmap) {
    Canvas canvas = new Canvas(baseBitmap);
    mAnimatedDrawableBackend.renderDeltas(frameNumber, canvas);
  }
Called from within {@link #renderFrame} to ask the caller for a cached bitmap for the
specified frame number. If the caller has the bitmap cached, it can greatly reduce the work
required to render the requested frame.

@param frameNumber the frame number to get
@return a reference to the bitmap. The ownership of the reference is passed to the caller who
    must close it.
animated-base/src/main/java/com/facebook/imagepipeline/animated/impl/AnimatedImageCompositor.java