Fix resource leaks discovered while testing

- `Drawable` now removes its `Skin` on `dispose`, disconnecting events.
- Creating a new `Drawable` doesn't always create a new `Skin` any more:
  now it can share an existing skin.
- Don't throw when asked to update the properties of a `Drawable` that
  has already been destroyed. This seems like a VM bug but was causing
  overwhelming output in the browser for some projects.

src/Skin.js

@@ -37,6 +37,8 @@ class Skin extends EventEmitter {
              */
             u_skin: null
         };
+
+        this.setMaxListeners(RenderConstants.SKIN_SHARE_SOFT_LIMIT);
     }
 
     /**