Optimized HBD block subtraction for all block sizes

- Interface function takes a local MxN function to call based on the
  block size.
- Repetition call (w/o cache line miss) shows improvement:
  ~63% - ~340%.
- Overall encoder speed improvement: ~0.9%.

Change-Id: Ieff8f3d192415c61d6d58d8b99bb2a722004823f
4 files changed