rvp8: add VP8 inter-frame decoding and a persistent video decoder

Extend the key-frame decoder to full RFC 6386 video: inter frame
headers, per-MB mode and motion-vector decode (ZEROMV/NEAREST/NEAR/
NEW/SPLITMV and intra-in-inter), near-MV prediction with reference
sign bias, six-tap and bilinear sub-pixel motion compensation,
version 0-3 reconstruction filters including full-pixel chroma,
per-mode/ref loop-filter level deltas in both loop filters, and
last/golden/altref reference management behind a new rvp8_video_*
persistent-decoder API.

Verified byte-identical to libvpx across 600 frames in 12 streams
covering all inter modes, all three reference frames with copy and
refresh juggling, multi-partition tokens, mid-stream key frames,
non-multiple-of-16 dimensions and profiles 0-3; ASan/UBSan clean.

Author: LibretroAdmin