fix: default property deserializer was missing for nested extras obje…

…cts in MeshPrimitive

fixes prefrontalcortex#126

Runtime/Plugins/GLTFSerialization/Schema/MeshPrimitive.cs

@@ -161,6 +161,9 @@ public static MeshPrimitive Deserialize(GLTFRoot root, JsonReader reader)
 									case "targetNames":
 										primitive.TargetNames = reader.ReadStringList();
 										break;
+									default:
+										primitive.DefaultPropertyDeserializer(root, reader);
+										break;
 								}
 							}
 						}