Colour MLS over all sources

See how well applying colour MLS filter to all sources works. It may be too slow but should produce good results when merging, unlike the individual colour MLS.

To optimize: Corresponding pixels are written back to at the same time and marked as done, meaning they are not then visited a second time when the next pair of images are processed? Or perhaps a set of flags is used to indicate which sources this pixel has already been smoothed with (32 bit int of source flags).