rc: use m_frameDuration instead of rce->frameDuration to derive complexity for each frame in 2nd pass.

don't store and use frameDuration from stats file per frame - losing precision.
search: re-enable chroma tskip

this passed regression testing
encoder: nits
copy_cnt_8, AVX2 asm code as per new interface, performance improved from 5.13x to 7.59x on HASWELL-I5
asm: avx2 assembly code for dct16

--HG--
extra : amend_source : c55d7f6dca4fbb22bd253bc238d45d02b2004f84
asm: enable copy_cnt8