Cycles: Make Blender importer more forward compatible

Basically the idea is to make code robust against extending
enum options in the future by falling back to a known safe
default setting when RNA is set to something unknown.

While this approach solves the issues similar to T47377,
but it wouldn't really help when/if any of the RNA values
gets ever deprecated and removed. There'll be no simple
solution to that apart from defining explicit mapping from
RNA value to Cycles one.

Another part which isn't so great actually is that we now
have to have some enum guards and give some explicit values
to the enum items, but we can live with that perhaps.

Reviewers: dingto, juicyfruit, lukasstockner97, brecht

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D1785

intern/cycles/blender/blender_mesh.cpp

@@ -827,7 +827,10 @@ Mesh *BlenderSync::sync_mesh(BL::Object& b_ob,
 
 	/* displacement method */
 	if(cmesh.data) {
-		const int method = get_enum(cmesh, "displacement_method");
+		const int method = get_enum(cmesh,
+		                            "displacement_method",
+		                            Mesh::DISPLACE_NUM_METHODS,
+		                            Mesh::DISPLACE_BUMP);
 
 		if(method == 0 || !experimental)
 			mesh->displacement_method = Mesh::DISPLACE_BUMP;