qt/mp4mux: write vpcC box for VP9 tracks where possible
commit d1f97896
mp4mux: enable muxing VP9 streams
As specified in https://www.webmproject.org/vp9/mp4/
commit ff8c9bc8
qtmux: add support for writing vpcC box for VP9
Increases compatibility for VP9 in .mov in at least VLC.
commit bc668b3c
vp9parser: initialize subsampling to -1
The default value of 0 is a valid subsampling value and could be
confused with an 'unset' value.