C++/gtImgMeanVal: updated version that outperforms matlab

By a factor 1.5 on big images, and by a factor of 2 on small images

This mainly served as a test bench to improve my SSE2 and OpenMP skills :) no real need otherwise.

Signed-off-by: Nicola Vigano <nicola.vigano@esrf.fr>

git-svn-id: https://svn.code.sf.net/p/dct/code/trunk@664 4c865b51-4357-4376-afb4-474e03ccb993