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.