Add Vapoursynth decoding support to allow for avoiding piping

[shssoichiro]

This is primarily for av1an, but may be useful to other applications as well. Currently, any application wanting to use this library must convert its input to y4m, and the simplest way to do this is to pipe from vspipe or ffmpeg. This change adds a Vapoursynth decoder, gated behind a Cargo feature (off by default), to allow library consumers to avoid this piping. This provides substantial performance improvements. The amount of improvement varies depending on the source video format, as this moves the bottleneck to be on the decoder, but tests showed anywhere from 20-75% improvement on 8-bit for the fast scenechange method, with smaller but still respectable improvements of up to 20% for 10-bit and for the standard scenechange method, compared to piping from vspipe.