Memory leak when using offscreen rendering

Hello,

When using offscreen rendering, there are 2 problems with memory
when destroying the 3D scene:

1. When invoking DisplayImplJ3D.destroy(), I get a 'IllegalStateException'
thrown in Canvas3D.stopRenderer().
Suggested solution: In the VisADCanvasJ3D.stop() method, instead of
calling stopRenderer(), the method removeNotify() should be invoked.

2. The UniverseBuilderJ3D.destroy() method is never reach, because the
attribute 'component' in VisADCanvasJ3D is null.
Suggested solution: Add a attribute 'universe' in DisplayImplJ3D, which
has e reference to the universe and is assign in the initialize(..)
method. When the DisplayImplJ3D.destroy() method is invoked, call also
universe.destroy();

Cheers,

Hannes