gsPointGrid use of integer types int and unsigned

[hverhelst]

In the gsPointGrid class, we make use of int and unsigned, while (almost?) everywhere else in the code, we use shirt_t and index_t. I changed it locally, but some tests break down (e.g. gsRemapInterface_test:MatchingAffineTest1.). I realized that the change of the integer types in gsPointGrid could have quite an impact throughout G+Smo. However, maybe it is worth the effort?