title: Result is ok at preview, but empty after rendering
body: <p>I want to cut this hemisphere:</p>
  
  <p><a href="https://i.stack.imgur.com/jHvQ0.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/jHvQ0.png" alt="enter image description here"></a></p>
  
  <p>my code is:</p>
  
  <pre><code>difference() {
  import("../kupol.stl");
  translate([-150,-1,-1]) cube(500);
  }
  </code></pre>
  
  <p>at preview result is ok:</p>
  
  <p><a href="https://i.stack.imgur.com/dDLEW.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/dDLEW.png" alt="enter image description here"></a></p>
  
  <p>but after rendering (F6 key) I received an empty object:</p>
  
  <p><a href="https://i.stack.imgur.com/8QQUe.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/8QQUe.png" alt="enter image description here"></a></p>
  
  <p>Rendering was finished without any errors:</p>
  
  <pre><code>Compiling design (CSG Tree generation)...
  Rendering Polygon Mesh using CGAL...
  Geometries in cache: 15
  Geometry cache size in bytes: 3178728
  CGAL Polyhedrons in cache: 12
  CGAL cache size in bytes: 0
  Total rendering time: 0 hours, 0 minutes, 0 seconds
  Rendering finished.
  </code></pre>
  
  <p>I'm using OpenSCAD version 2015.03-2 at Windows 7 64bit machine.
  <a href="http://paste.ubuntu.com/14480974/" rel="nofollow noreferrer">Here</a> is detailed information about my build and <a href="https://drive.google.com/file/d/0B6eZzxZXMc-BYm9idXdEQU4xMkU/view?usp=sharing" rel="nofollow noreferrer">here</a> is stl with hemisphere which I need to cut.</p>
  
  <p>What am I doing wrong?</p>