revert to boost::geometry point type, but using IV coord type

Point coords for boost::polygon::voronoi input should be 32 bit ints.
For genereral boost::geometry points, we want to support whatever
the perl int type supports (IV type in C++ code), which will often
be 64 bits. So using boost::polygon point_data type as the main type
for both libraries, while possible, is not ideal at this point.