Support RGB/CMYK images with bits per component < 8

[felipeochoa]

There's already a commented-out section of code for roughly dealing with images where the BPC is 2 or 4. However, there may be a more efficient way by

1. Submitting an upstream (i.e., PIL) patch to support these use-cases; or
2. Using the existing 1, L;2, and L;4 modes to expand the the data and then using tobytes and
   frombytes