fix bug: valid json will failed when object template contains zIndex (#698)

config/schemas/objectTemplate.json

@@ -38,6 +38,10 @@
 			"type":"array",
 			"items": { "type": "string" },
 			"description": "Object mask that describes on which tiles object is visible/blocked/activatable"
+		},
+		"zIndex" :  {
+			"type":"number",
+			"description": "Defines order in which objects on same tile will be blit."
 		}
 	}
 }