#432: VP8 video decodes (clean-room pure-Rust VP8 decoder)

VP8 WebM showed black video: there was no VP8 decoder. New L0 crate
filmcraft-vp8, implemented from RFC 6386 sections 1-19 (tables extracted
from the RFC text, nothing from the attached reference decoder): every
coding tool, versions 0-3, hidden altref frames, size changes. Bit-exact
with libvpx on 62 libvpx VP8 test vectors, real-world FATE samples and
generated fixtures; never panics on corrupted input.

Wired into codecs for Matroska / WebM (V_VP8, including the alpha layer
of transparent WebM) and MP4 (vp08). VP8 colour is BT.601 (RFC 6386 9.2).
A hidden altref frame is a WebM block of its own with the time of the
frame that follows it: the VP8 time lookup now moves on to that shown
frame instead of repeating the previous one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

xtask/src/main.rs

@@ -44,6 +44,7 @@ const LAYERS: &[(&str, u8)] = &[
     ("h264", 0),
     ("h264enc", 0),
     ("hevc", 0),
+    ("vp8", 0),
     ("vp9", 0),
     ("prores", 0),
     ("mjpeg", 0),