Update TextureBGS.cpp

the operator () is invoked with arguments (x, y) but in
https://github.com/andrewssobral/bgslibrary/blob/master/package_bgs/dp/Image.h#L322
the operator () assumens (row, colum):
// pixel-level access using image(row, col)
inline unsigned char& operator()(const int r, const int c)
so the correct invocation should have been with (y, x)