Rawspeed CHDK raw support

Turns out the CHDK guys decided a good raw format was "a file with nothing but the raw data in it". dcraw detects these files by their exact file size. Brittle but works. This adds a decoder for those files. It works fine but the color matrix isn't being applied as darktable can't find make/model. Another case (like the MOS files) where switching to rawspeed for determining make/model would be better.