Add non-negative constraint to the pre/post filter search tool.

This avoids some waffling effects at the expense of some coding gain.
tools: Fix sign comparisons and check fread/fwrite
encoder: fix braces and indentation
Respect OD_DUMP_IMAGES_SUFFIX during decode.
Change from cut to awk for OS X.
 Add VPX support to rd_collect tools.
Dubious derivation for quantization distortion using a biorthogonal transform