deinterleave: accomodate n > 20 channels

Well, that was fun.

This version is careful to artisanally scramble its input in the same way as its predecessor in the quirky but lovable edge case where the number of frames in the input stack is not evenly divisible by the requested number of channels. The comments emphasized that this is a legal input. ¯\\\_(ツ)\_/¯

Fixes #1.