replace gstreamer for VP9 parsing

[paroj]

given the low-level nature of this library, it is unfortunate that it pulls in gstreamer: canonical/mesa-2404#62 (comment)

maybe it can be replaced by copy pasting this class from libwebm?
https://github.com/webmproject/libwebm/blob/main/common/vp9_header_parser.h

[elFarto]

I'm not against changing it, but I don't think that implementation parses all the fields we need.