Mixed bit depth

[chrstphrbrns]

The code currently assumes uniform bit depth for all channels, but the TIFF format seems to support mixed-depth channels, given that the BITSPERSAMPLE tag stores an array for multi-channel data

Implementation would require support from ColorTypes and changes to the recode, deplane, and prediction logic

[tlnagy]

Seems like #159 goes a way towards making this a reality? I think this would be great to have.