FIX: Allow as_grey on RGBA images.

Fix bug in PIMS that broke `as_grey` for images with an alpha channel.
from mpl work, alpha compositing turns into a can of worms and is rather subtle to get right.  I this case I think we should composite it down on to a white background as just dropping the alpha channel can lead to bad things (such as (0, 0, 0, 0) turning into black).