more demosaic tuning for x-trans

- don't use Bayer-only edge-aware median for < 50% scaled exports
- hide Bayer-only options in GUI
DMC-GH1 aspect ratio for comparison testing
use libraw_dcraw_process() again

It figures out proper filters for Bayer images, deals with weird hot
pixels in x-trans images, and crops the image. It also allocates lots
of extra memory and copies the image one time more than necessary.
Also get rid of unused variable image, left over from older LibRaw
interface.