This is a tiny sliver of NihAV stripped out: just the VP5/VP6 decoder, and a couple related bits and pieces to make it work.
NihAV is a research multimedia framework with some demonstration tools.
This portion of NihAV is kindly relicensed by the author to be used by the Ruffle project under the MIT License - see COPYING for details.
As also noted on: https://codecs.multimedia.cx/nihav-relicensed-code/