Add mp4 support
Since the mp4 format has some mandatory metadata, I don't know what to do with them. Currently, I'm hiding all the irrelevant/nullified ones, but some are remaining:
$ ./mat2 -s ./tests/data/dirty.cleaned.mp4
[+] Metadata for ./tests/data/dirty.cleaned.mp4:
CompatibleBrands: ['isom', 'iso2', 'avc1', 'mp41']
CompressorID: avc1
GraphicsMode: srcCopy
HandlerDescription: SoundHandler
HandlerType: Metadata
HandlerVendorID: Apple
MajorBrand: MP4 Base Media v1 [IS0 14496-12:2003]
MediaHeaderVersion: 0
MinorVersion: 0.2.0
MovieDataOffset: 48
MovieHeaderVersion: 0
NextTrackID: 3
PreferredRate: 1
Rotation: 0
TimeScale: 1000
TrackHeaderVersion: 0
TrackID: 1
TrackLayer: 0
I'm a bit afraid that users will think that this is a bug, since mat2 is supposed to remove metadata; but on the other hand, I'm not comfortable hiding scary values like "HandlerVendorID: Apple", even if it's only ffmpeg using funky "default" values.
Thoughts?