Support 64bit integer properly

The example code utilizes 64bit integer, but it does not support 64bit integer actually.

For instance, slope comparison is done with multiplication of two coordinate delta and therefore, one if each coordinate value exceeds 32bit range, it causes overflow.
Original Clipper utilizes 128bit integer operation to compare slope.

Locate the example code's slope comparison and inspect how multiplication of coordinate deltas can overflow for values beyond the 32-bit range. Compare that operation with the original Clipper's 128-bit integer approach; done means slope comparisons remain correct for 64-bit coordinate values without overflow.

Written by the indexing model from the issue text.