[x264] Add b_pyramid setting

[everling]

Stanley, can you add the 'b_pyramid' setting? Despite it's current dubious status, it does help in x264 encoding quality.

b_pyramid: You might as well enable this option if you are using >=2 B-frames; as the man page says, you get a little quality improvement at no speed cost. Note that these videos cannot be read by libavcodec-based decoders older than about March 5, 2005.

This usually results in slightly improved compression, at almost no speed cost. However, this is an experimental option: it is not fully tuned and may not always help. Requires bframes >= 2. Disadvantage: increases decoding delay to 2 frames.

[stanley]

OK, I will add it.