VDecimate: A bit of refactoring
VDecimate: A bit of refactoring

Fewer magic numbers, more C99, less code duplication.

vapoursynth/vapoursynth,Kamekameha/vapoursynth,Kamekameha/vapoursynth,vapoursynth/vapoursynth,vapoursynth/vapoursynth,Kamekameha/vapoursynth,Kamekameha/vapoursynth,vapoursynth/vapoursynth