* Optimize the self-guided and domaintxfmrf filters
* Save 576KiB of buffers in the encoder and decoder
* Disable self-guided filter for videos whose width or
  height is < 5, in order to help simplify the filter.

This results in an overall 30-40% improvement in decoder
speed with loop-restoration enabled (depending on source
and bitate), with no effect on video quality, *except* for
videos with width or height < 5 pixels.

Change-Id: Ide9181118ec3a63a0335338f316505b08df2d831