Bugfix: scene guides were drawn during object selection and there was a

missing check for zero-size name-stack size. So, hovering over an axis or
camera center sphere resulted in a crash.

* TGLSelectBuffer - return name-stack size from Set() method.
* TGLViewerBase - check name-stack size before accessing elements.
* TGLViewer - do not draw non-scene objects in rendering for selection.


git-svn-id: http://root.cern.ch/svn/root/trunk@43202 27541ba8-7e3a-0410-8455-c3a389f83636