Fix compile error in Debug mode. Adds getters for Point3 types in ExtrusionEntity
ZAA changed ExtrusionPath::polyline from Polyline to Polyline3, preserving the existing interfaces by converting first_point and last_point to return a Point copy constructed from the underlying Point3 type.
ExtrusionLoop::validate function was not updated and is broken in debug configurations as it's currently comparing Point to Point3
This change promotes ExtrusionPath::first_point3/last_point3 to the ExtrusionEntity base class as a pure virtual function, implements them on derived classes, and fixes ExtrusionLoop::validate
