Partially fix OpenCL/gegl:noise-reduction
This commit addresses the sign flipped by OpenCL implementation causing a lot of wrong pixels in gegl:noise-reduction. The code has been updated with select statements to potentially be optimised at compile time for vectorized performance.
Addresses part of #162. The operation is still not fixed in the non-inplace version due to some other issues.
Edited by Varun Samaga B L