Timur:
1. Allow unique identification of shared GL contexts via
TGLContextIdentity class.

Matevz:
1. Improved overlay selection handling.
2. Perform GL context check before rendering, possibly invalidating
   registered display-lists.
3. Minimize updates for window expose and resize events.


git-svn-id: http://root.cern.ch/svn/root/trunk@19049 27541ba8-7e3a-0410-8455-c3a389f83636