title: GLCanvas inside JPanel doesn't work
body: <p>I am trying to add a GLCanvas with OpenGL-Content to a JPanel. The JPanel is inside a JTabbedPane. But when the GLCanvas is inside the JPanel, the Panel is just grey. When I add the GLCanvas directly into the TabbedPane, everything works fine.</p>
  
  <p><img src="https://i.imgur.com/Jd5qx11.png" alt="xxx"></p>
  
  <p>Here the working code:</p>
  
  <pre><code>    JTabbedPane mainPane = frame.getMainPane();
      GLCanvas canvas = cogl.getCanvas();
      mainPane.add("OGL",canvas);
  </code></pre>
  
  <p>An here is the not-working code:</p>
  
  <pre><code>    JTabbedPane mainPane = frame.getMainPane();
      GLCanvas canvas = cogl.getCanvas();
  
      JPanel panel = new JPanel();
      panel.add(canvas);
  
      mainPane.add("OGL",panel);
  </code></pre>
  
  <p>So how can i get the GLCanvas working inside the JPanel?</p>