SC downscale further performance improvements

A bit of a fuzzy issue since I'm not entirely sure how much improvement we can get. It seems like although SC downscale is providing some performance improvements, it is not nearly as much as one would hope for. The cause is that the downscaling is bottlenecking the scenechange detection.

We should do some investigation to see if there are faster alternatives to Vapoursynth's bicubic that we could use.