Improved filter size

Similarly good results as with the 31x31 filters can indeed be obtained with smaller, asymmetric filters.

A darktable fork serving as proof-of-concept can be found here: https://github.com/ILiebhardt/darktable.git.

For trying, just compile, deactivate openCL (only C code thus far), and choose ‚1 pass Markesteijn‘ as demosaicking method (doesn’t work for 3-pass, and wouldn’t really yield advantages, either).