mapping binding - never overwrite already set item->instance

1) this way code should be more straightforward
2) this allows us not to reimplement mapping.get_surface_style logic in IfcGeomWrapper as it will be already handled in map_impl(const IfcSchema::IfcStyledItem* inst) and the resulting item->instance is not overridden by IfcStyledItem.

src/ifcgeom/mapping/bind_convert_impl.i

@@ -8,7 +8,7 @@
 		try { \
 			item = map_impl(inst->as<IfcSchema::T>()); \
 			if (item != nullptr) { \
-				if (!(item->instance && inst->as<IfcSchema::IfcMaterial>())) { \
+				if (item->instance == nullptr) { \
 					item->instance = inst; \
 				} \
 				try { \