Implement vertex id

This enables the library user to map the output vertices in the final
triangulation/partition back to the input vertices.  This allows
additional vertex attributes to be mapped from the input vertices onto
the output triangle vertices in a straightforward way.

src/polypartition.h

@@ -33,6 +33,9 @@ typedef double tppl_float;
 struct TPPLPoint {
     tppl_float x;
     tppl_float y;
+    // User-specified vertex identifier.  Note that this isn't used internally
+    // by the library, but will be faithfully copied around.
+    int id;
 
     TPPLPoint operator + (const TPPLPoint& p) const {
         TPPLPoint r;