Don't force row QPs to integer values with VBV

Don't force row QPs to integer values with VBV
VBV should no longer raise the bitrate of the video.  That is, at a given quality level or average bitrate, turning on VBV should only lower the bitrate.
This isn't quite true if adaptive quant is off, but nobody should be doing that anyways.
Also may result in slightly more accurate per-row VBV ratecontrol.