[x265] recent chroma residual improvements

[Steve Borho]

Over the past couple weeks there have been a number of improvements in
the handling of chroma residual.

Deepthi recently made presets medium and slow (--rd levels 3 and 4)
consider the cost of chroma residual when making non-RDO (SA8D)
decisions. So for presets medium and slow it is now considering the
chroma residual cost when:

* selecting the best inter CU partition mode
* selecting the best merge candidate
* selecting between mv based bidir and colocated bidir [1]

The choice between the best inter mode, 2Nx2N bidir, 2Nx2N merge,
or intra is still an RDO decision at these presets.

I just pushed support for chroma residual considerations in SATD
decisions, which is enabled at --subme levels greater than 2.
Example SATD decisions:

* subpel refinement of motion vectors
* deciding between reference pictures
* deciding between merge, unidir, and bidir prediction

SATD decisions are made at all RD levels and so considering chroma
residual here improves visual quality even for the presets that already
used all rate-distortion optimized mode decisions. Preset slow is the
first preset to use --subme 3 or higher, so it is the first preset to
enable chroma SATD by default.

I've updated the online docs at http://x265.readthedocs.org/en/default/

Let me know if you experience any problems with it.

-- 
Steve Borho

[1] there was a bug here just fixed yesterday