Restructure ffmpeg

[WithoutPants]

Significant refactor of the ffmpeg package. Focus here was on making a more consistent and reusable package interface.

Changes include:

• removed the stripExt parameter from NewVideoFile since this was used during scanning only
• introduced fsutil.ReadLockManager to replace running stream checking code. Allows clients to "lock" a particular file for reading. When an operation requires to delete a file, it runs Cancel on all of the running lock contexts before returning to allow deletion.
• eliminates ffprobe execution when stream transcoding files. Should improve performance.
• moved frame rate calculation code into ffmpeg
• moved most scene generation code into new scene/generate package
• added ffmpeg/transcoder package to expose basic video and image manipulation functions: Transcode (re-encodes or copies a video with optional start time and duration), Splice (combines video files using a concat file), Screenshot (takes a screenshot at a given position), ImageThumbnail (makes a thumbnail of an image).
• moved JSONTime into models/json package so that ffmpeg does not depend on models.
• moved video phash generation code into new hash/videophash package. This should help formalise the algorithm.
• added functions to ffmpeg for argument building

Most public interfaces are now documented according to godoc specs. The only thing that's still in a halfway state is the sprite generation stuff, which will need another refactor pass in future.

Biggest things to consider with this change are ensuring phash generation is consistent with the existing behaviour, ensuring all generation operations work correctly, ensuring live transcoding continues to work and that scene deletion follows existing behaviour with respect to running streams.

[bnkai]

PR seems ok and tests ok.

Edit: Updated after b7cabfa
Checked

• previews generation
• screenshots generation
• markers generation
• phashes generation (phash values seem to remain the same as before for a few scenes i checked)
• live transcoding seems to work ok
  Cancelation also seems to work if you try to delete a scene that is live transcoded

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Todo ?

• Fix 10bit vp9 streaming #2458 needs to be re-applied (it is probably in the conflicting files)
• According to Hoist context, enable errchkjson #2488 context should not be used in structs, so the lock_manager code may need some refactoring?