Add function to compute distance between cur and ref
Add function to compute distance between cur and ref

The function get_ref_frame_dist will return the number of
frames between the current frame and one of its references.

Change-Id: I0d507975c6bb5acd8f986e67e0499cb9fea0cce9

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom