Add Frame Seeking to VapourSynth and FFMS2 Decoders

[BoatsMcGee]

Adds a way to skip frames by setting the frames_read value in the Decoder. Allows decoding a subset of the video without having to decode the frames prior to the requested frames. Particularly useful for Scene Detection, where implementing pause/resume would nullify the gain from reduced computation because all frames had to be decoded (see example below).

Also enables seeking in the FFMS2 decoder by setting the seek mode from 0 (linear) to 1 (normal). I don't know why this wasn't implemented in the first place or maybe it was overlooked.

Example

// Frame 14000 was already detected in a previous session. Consider it "read" in this new session.
decoder.seek_to_frame(14000)?;
// av-scenechange will decode from frame 140001 instead of 0, eliminating redundant/unnecessary decoding
detect_scene_changes::<u8>(decoder, options, None, None)?;