Move enums away from global namespace:

- EOverlap is now in namespace Rgl;
- EClipType is now in class TGLClip;
- EPosition and EManipType were not used and have been removed.

EGLCoordType and EGLPlotType are left in top-level. In these two enums all
names begin with kGL so there is less chance for collision.


git-svn-id: http://root.cern.ch/svn/root/trunk@46574 27541ba8-7e3a-0410-8455-c3a389f83636