Save the colour model and channels from the original image so the preview is the same i.e we make greyscale previews from greyscale scans.
Add a "for description" string to Destination::get_profile() so we can pass it along to Image::default_profile().
It's now a little clearer what the default profiles are being chosen for.
Oops, include sample.h since we're not defining SAMPLE in the makefile anymore.