demux/mkv: assume VP9 is packetized in Matroska/WebM
This effectively reverts the fix from 17e07afc.
The issue in #12944 (closed) was that libavcodec was not handling super frames. It was fixed in https://github.com/FFmpeg/FFmpeg/commit/a5679933c1b8b6bef5c5c3eb7c70d06c695066cf which is in lavc 58.6.102. We require 58.54.100.