support alpha in VP8/VP9 WebM
Alternative version of !3600 (closed) with less FourCC (only one for each alpha bitstream that is not decodable by a regular VP8/VP9 decoder, at least for VP8).
The i_level value is currently not used by VP8/VP9 so a flag is set in there to detect regular VP8/VP9 from VP8/VP9 with alpha.
IMO it impacts way too much code to avoid decoding VP8/VP9 with alpha in decoders that will just discard it.