Fix inconsistent creation of NodeTreeTypeUndefined type of node tree

Currently, when creating a new node tree ID, its `typeinfo` is set to
`NodeTreeTypeUndefined`, but its `type` enum value is left to `0`,
aka `NTREE_SHADER`.

This patch adds a new `NTREE_UNDIFINED` value, and use it for
`NodeTreeTypeUndefined` types of node trees.

NOTE: While it is not clear whether that actually fixes issues currently,
quite a bit of code still relies on the value of `type`, so think it
makes sense to sanitize this.

NOTE: Would have been ideal to reserve `0` value to undefined type,
but at this point this is not possible anymore, so chose to use `-2` instead.

Reviewed By: JacquesLucke

Differential Revision: https://developer.blender.org/D13123

source/blender/blenkernel/intern/node.cc

@@ -1160,15 +1160,15 @@ static void ntree_set_typeinfo(bNodeTree *ntree, bNodeTreeType *typeinfo)
 {
   if (typeinfo) {
     ntree->typeinfo = typeinfo;
-
-    /* deprecated integer type */
-    ntree->type = typeinfo->type;
   }
   else {
     ntree->typeinfo = &NodeTreeTypeUndefined;
 
     ntree->init &= ~NTREE_TYPE_INIT;
   }
+
+  /* Deprecated integer type. */
+  ntree->type = ntree->typeinfo->type;
 }
 
 static void node_set_typeinfo(const struct bContext *C,
@@ -5533,6 +5533,7 @@ static void register_undefined_types()
    * they are just used as placeholders in case the actual types are not registered.
    */
 
+  NodeTreeTypeUndefined.type = NTREE_UNDEFINED;
   strcpy(NodeTreeTypeUndefined.idname, "NodeTreeUndefined");
   strcpy(NodeTreeTypeUndefined.ui_name, N_("Undefined"));
   strcpy(NodeTreeTypeUndefined.ui_description, N_("Undefined Node Tree Type"));