sad avg: align intermediate buffer
sad avg: align intermediate buffer

comp_avg_pred has started declaring a requirement for aligned buffers.

BUG=webm:1390

Change-Id: Idaf6667498ea343e8d49b32bc9d8b9d0aa43ef5c

webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx