Merged log application to precomp contribs when copying to/from matrix in preComputedContrib() resp. getPreCompContribByPhoton(). In the linear case these are memcpy'd per row. In the log case these have to be log'd and assigned per element which is slow. Coefficient indices are only calculated for rows in the outer (row) loop and only incremented in the inner (column) loops to reduce multiplications. A coefficient pointer is used similarly thresholdContribs().