Update libvpx to 1.8.2
This release collects incremental improvements to many aspects of the library.
- ARCH_* defines have been removed in favor of VPX_ARCH_*.
- Used ffmpeg to transcode H264 video to VP9 with libvpx-vp9 encoder.
- Decoded resulting video to Y4M with vpxdec.
Reviewers: Triage Team, JoshStrobl
Reviewed By: Triage Team, JoshStrobl
Differential Revision: https://dev.getsol.us/D9177