demo: Add polygonOffsetFactor for debug visualization

Constant offset is not quite enough to make sure the visualization lines
cleanly render on top of solid geometry.

demo/simplify.html

@@ -339,6 +339,7 @@
 								// use small depth offset to avoid overlay z-fighting with the original mesh
 								// has to be done on the main material as overlays use lines that don't support depth offset
 								object.material.polygonOffset = true;
+								object.material.polygonOffsetFactor = 0.5;
 								object.material.polygonOffsetUnits = 16;
 
 								object.material = [