local laplacian: make roi aware in darkroom mode
this only affects zooming in darkroom mode. the upper level of the laplacian pyramid are stolen from the preview pipeline. because they are at a different resolution, trilinear filtering is done to braid the data into the new output pyramids. this can result in some artifacts on very fine geometry for unlucky ratios between full and preview resolution. i think it's already a lot better than without though. the opencl code path is not fixed yet. i did not benchmark it, but since the full pipeline actually computes /less/ it should be faster modulo synchronisation with the preview pipe.
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