libvpx

Do horizontal loopfiltering in parallel

This patch followed "Rewrite filter_selectively_horiz for parallel
loopfiltering" commit, and added x86 SSE2 optimization to do
16-pixel filtering in parallel. Also, corrected the declaration
of aligned arrays. For 8-pixel-in-parallel case, improved the
calculation of the masks and filters. Updated the threshold loading
since the thresholds were already duplicated. Updated neon C functions
to call neon loopfilters twice.

Using tulip clip, tests showed it gave a ~1.5% decoder speed gain.

Change-Id: Id02638626ac27a4b0e0b09d71792a24c0499bd35