MP4: Use hdvd atom to detect videos (requires mutagen >= 1.43)

Apple supports a hdvd atom in MP4, which is a 8 bit integer where:

0 - SD
1 - 720p
2 - 1080p

We can use the presence of this atom to identify a file as video.

This requires mutagen 1.43, see https://github.com/quodlibet/mutagen/pull/386