Remove unnecessary fdct and other routines
diff-stream.cc: no need for display
rewrite_as_diff takes full DecoderState
Update the entire ProbabilityTable on a diff frame
On diff frame, update only the changed probability table entries
Update y_mode and uv_mode probs in diff frame header
MVProbUpdate: Simplify get() method
Match motion_vector_probs using frame header
Wipe segmentation map if segmentation not enabled on this frame (thanks to BBB)