x86: faster AVX2 iDCT, AVX deblock_luma_h, deblock_luma_h_intra
Tweak i16x16-delta-quant-avoidance code

Don't omit the delta quant if it'd raise the quantizer to do so; this fixes
a rare flickering issue caused by deblocking.
Interface: if vbv-maxrate < bitrate, set bitrate = vbv-maxrate

This probably makes more sense to the user than setting vbv-maxrate = bitrate,
as before.