GIL TIFF and CMYK fixes

A couple of fixes:
• change from * to channel_multiply in the default colour converter for CMYK -> RGB.
• Automatically infer photometric_interpretation tag from the image colour type /unless/ the user has manually specified one (I /think/ the original just has inverted logic here).

These two bugs together allow the loading (and conversion into e.g. RGB) and saving of CMYK TIFF files to function correctly (neither was working before!)