proper vfr handling

[f0e]

At the moment VFR clips are converted to CFR with the framerate being that of the video file, with duplicate frames being added where required. This means those frames need to be deduplicated, which sucks. Ideally you could process the VFR clip properly in VapourSynth, removing the need to deduplicate, and then restore the proper timestamps when encoding in FFmpeg. However, FFmpeg doesn't seem to have great support for this