int addvertex(int geohandle, int prim_num, int point_num)
A handle to the geometry to write to. Currently the only valid value is
0 or geoself, which means the current geometry in a node. (This argument may be used in the future to allow writing to other geometries.)
The primitive number to add the vertex to.
The point number to wire the new vertex to.
Returns a linear vertex index, or
-1 if the vertex could not be added. You can use this number with setvertexattrib to set attributes on the new vertex, however this number may not be the final vertex index.