Add grid checkbox

index.html

@@ -52,6 +52,13 @@
         <span id="display-fps">12 fps</span>
       </li>
     </ul>
+
+    <ul class="tools-group">
+      <li class="tool-grid">
+        <input id="show-grid" type="checkbox"/>
+        <label for="show-grid">Show grid</label>
+      </li>
+    </ul>
   
   </div>