vbv: bug fixes to stop mid frame qp adjustments to increase erratically.

maintain bufferFill for each FrameEncoder context and use that in rowWise qp adjustments for vbv.
also fixes bugs in predicting row wise bits from satd costs.

--HG--
branch : stable
param: add more validation checks to prevent encoder crashes

--HG--
branch : stable
asm: 10bpp code for vertical luma interpolation filters.
Merge with stable
api: expose X265_BFRAME_MAX to the user
log: report consecutive B-frame histogram, as x264 does
asm: cleanup 444 chroma primitive setup, and other primitve reuse