std: fix SplitPlanes
std: fix SplitPlanes

ShufflePlanes takes a `planes` argument, not `plane`.

Signed-off-by: akarin <042dc4512fa3d391c5170cf3aa61e6a638f84342@akarin.info>

vapoursynth/vapoursynth,vapoursynth/vapoursynth,vapoursynth/vapoursynth,vapoursynth/vapoursynth