vpnext: pick loop filter segment by segment

Picks a per segment loopfilter.  Adapts the algorithm to search for
a loopfilter value for each separate segment.  Further todo fix the
bias

Improvements .06 % ov psnr, .11% ssim
http://www.corp.google.com/~jimbankoski/no_crawl/segmentedpicklpf.html

Change-Id: Ic6a571c16fcd6ec0139f4de1f8061f87c6515a10