Fix incorrect calculation of local contrast adaption in color edge detection

[iRi-E]

In SMAAColorEdgeDetectionPS(), the calculations of left-left and top-top deltas are performed as:

    t = abs(C - Cleftleft);

and

    t = abs(C - Ctoptop);

However, these are not differences of neighboring pixels, so should be modified as follows:

    t = abs(Cleft - Cleftleft);

and

    t = abs(Ctop - Ctoptop);

Thanks!