video: pick up VP6-with-alpha decoder fix

Bumps nihav-vp6 to ruffle-rs/nihav-vp6#3, which fixes a 3-byte off-by-one
in the VP6-alpha color sub-stream length. Adds a regression test that
decodes a real VP6-alpha keyframe end-to-end.

Fixes #23662.

Author: appmana-accounts