Fix for zero sized ellipse object in tilemap

Indeed, in _Tiled_, ellipse can be zero sized.

I use this feature to place marks on the map where dynamic objects will be added at runtime.

For exemple, in one of my map, I place coins in the map:

```XML
[...]
 <objectgroup name="coin" width="64" height="48">
  <object x="511" y="663">
   <ellipse/>
  </object>
  <object x="531" y="663">
   <ellipse/>
  </object>
[...]
 </objectgroup>
[...]
```