Loren Merritt authored Apr 25, 2013 and Michael Natterer committed May 10, 2013

Adjust over-relaxation factor as a function of problem size.  Remove
the second array, and update in-place.  Factor branches and indexing
out of the inner loop, instead precompute a list of pixels inside the
brush mask and what neighbors they have.  Switch from scalar double to
simd float.

Speedup (of the laplace part, excluding gamma correction): 10x-20x,
depending on brush size.