Fixed some instance of simplify_polygons() invocation.

Geometry::deg2rad() made a template.
Some methods of Layer made inline.
Added a helper template remove_nulls().

xs/src/libslic3r/ExPolygon.cpp

@@ -198,7 +198,7 @@ ExPolygon::simplify_p(double tolerance) const
         p.points.pop_back();
         pp.push_back(p);
     }
-    simplify_polygons(pp, &pp);
+    pp = simplify_polygons(pp);
     return pp;
 }