fast decode tune option

[Steve Borho]

x264 has a --tune option that makes the encoded stream very efficient to decode; ie: it doesn't use any features that cause decoder bottlenecks.

We should add the same feature to our encoder. Steps:

1. look at how x264's fast decode tuning works, be sure to copy their tuning name
2. add this tune option to x265_param_default_preset()
3. add the option to the --tune CLI help, and our documentation

Our fast decode tune should set at least these options:

• --no-weightp
• --no-sao
• --no-lft
• --no-intra-in-B (by clamping --rd, I guess)
• --max-merge=1
• --wpp