Allow loading gltf mesh without material

Author: einarf

demosys/scene/loaders/gltf.py

@@ -403,7 +403,7 @@ def load(self, materials):
             bbox_min, bbox_max = self.get_bbox(primitive)
             meshes.append(Mesh(
                 self.name, vao=vao, attributes=attributes,
-                material=materials[primitive.material],
+                material=materials[primitive.material] if primitive.material else None,
                 bbox_min=bbox_min, bbox_max=bbox_max,
             ))