Add VideoFrameRate accessor to AVAssetTrack via Extension

[vade]

Pretty minor, but it might be nice for a mild refactor of VideoFrameRate on AVAsset

• Provide a AVAssetTrack extension which provides a VideoFrameRate view
• Refactor AVAsset's extension to iterate on the AVAssetTracks to find the first VideoFrameRate

I agree if this feels a bit nit picky, but working on EDL file formats like OTIO / FCPXML / FCP7 XML its nice to have a cleaner interface.

Thanks for considering, and btw, this library is AMAZING.

[orchetect]

Provide a AVAssetTrack extension which provides a VideoFrameRate view

Certainly something that could be added. I don't recall exactly why it wasn't added when the AVFoundation stuff in TimecodeKit was being built out. Either an oversight, or maybe there was some reasoning.

Refactor AVAsset's extension to iterate on the AVAssetTracks to find the first VideoFrameRate

That is already the current algorithm for both videoFrameRate(interlaced:) and timecodeFrameRate(drop:) on AVAsset. It iterates on tracks until it finds frame rate information that can be extracted. Are you running into a scenario where it fails to extract frame rate, or extracts the wrong rate?

[vade]

Oh neither! I just meant that it could be cleaned up a smidge with the new track based VideoFrameRate accessor is all. :)