The recursive partition type search is enabled down to 4x4, 4x8 and
8x4, followed by the corresponding rate-distortion optimization for
the per-partition encoding mode decisions.

The bit-stream writing/reading synchronized in supporting the
rectangular partition of 8x8 block.

This provides above 1% coding performance gains on derf.

To do next:
1. re-design the rate-distortion loop for inter prediction below 8x8.
2. re-design the rate-distortion loop for intra prediction below 4x4.
3. make the loop-filter aware of rectangular partition of 8x8 block.
4. clean the unused probability models.
5. update default probability values.

Change-Id: Idd41a315b16879db08f045a322241f46f1d53f20