asm: luma_vpp[64x32, 64x48, 64x64] in avx2: improve 14150c->9810c, 21132c->14684c, 28663c->19616c
asm: luma_vpp[64x16] in avx2: improve 7245c->4910c
Luma_hpp[32x8 , 32x16 , 32x24 , 32x32 , 32x64] avx2 asm code: improved 2032c->1556c, 4238c->3014c, 6696c->4801c, 8697c->6433c, 16823c->12297c
asm: luma_hpp[8x4] in avx2: improve 357c->261c
Luma_hpp[48x64] avx2 asm code : improved 25053c->17882c
asm: luma_hpp[8x8, 8x16, 8x32] in avx2: improve 623c->523c, 1384c->1083c, 2555c->2058c
slicetype: fix vbv lookahead data collection for all frames within the lookahead window.
rc: tune midframe vbv logic for B frames
rc: use c-style typecasts
rc: limit bit amortization in ABR to longer sequences
rc: adjust qp for B frames from ABR feedback in case of CBR.

limits the bitrate fluctuation for CBR with respect to the target bitrate set.
rc: improve the frame size planning with respect to vbv buffer occupancy and the lookahead window.
rc: introduce cli option to tune for cbr.
doc: add cbr to the list of tunings, add helpful cross-refs
doc: improve readability of film grain section