tree: check joint parent order

tree.py

@@ -7,10 +7,16 @@ def build_tree(gltf):
         if "children" not in node:
             continue
         for child_ix in node["children"]:
-            assert child_ix > node_ix, (child_ix, node_ix)
             assert child_ix not in parents
             parents[child_ix] = node_ix
 
+    for skin in gltf.json["skins"]:
+        seen = set()
+        for joint in skin["joints"]:
+            parent = parents[joint]
+            assert parent in seen or parent not in skin["joints"], (parent, joint, seen)
+            seen.add(joint)
+
     return parents
 
 if __name__ == "__main__":