fix photo::cloning::IlluminationChange when IPP is used

In the discussion of PR #3268, it appears that, under cerain circumstances, the cloning method IlluminationChange could fail and generate black images (except for a 1 px widr border).
This failure was triggered only when IPP librairies were used. It appears that cv::pow yields to different results depending on the implementation.
Most notably, the default implemtation considers that 0^(negative float) is 0, where the IPP implementation yields NaN.

The proposed patch fixes this by applying a mask to the relevant parts of the image.