Replace copyMode switch with fillTile function

It's even templated! Vapoursynth, here we come.
Replace separate GetFrames with generic functions

Not entirely there yet, since they still use Avisynth specific methods
of the VideoInfo class, but I'm one step closer nonetheless.
Remove extraneous TurnsTile cruft
Add direction to TurnsTile::mod

Will allow mod's use in processFrame functions, for easily calculating
ctr variables with other chroma subsampling values.