Major restructuring of GL viewer-scene relationship (Matevz)
===================================================

The main idea was to generalize current GL infrustructure so that the
following features can be done in the future:

1. Full pad rendering via GL including 3D histo/func painters and 2D graphics.

2. Implementation of GL canvas (with independent viewers serving as pads).

3. Multi-view event-visualization front-end.

4. Inclusion of external scenes (non-ROOT-gl) in ROOT viewer.
   Rendering of ROOT scenes in external GL frameworks.


Changes on the code level
-------------------------

0. Extend current scene/viewer classes into a 2-level class hierarchy.
   New classes TGLViewerBase and TGLSceneBase.

1. Allow scenes to be shared among viewers. View-specific scene-cache
   needs to be kept at the viewer side.

2. Viewer now manages a list of scenes. It is possible to put together
   a combination of scenes (e.g. one showing geometry, another one tracks
   and yet another clusters). Each scene can define its own clipping object.

3. Provide API for fine-grained control over rendered object removal,
   creation and update. This allows for efficient scene-updates.

4. Disentangle selection, clipping and other markup objects from
   within viewer-scene singleton.
   New classes TGLClipSet, TGLManipSet.

6. Disentangle selection management.

7. Separate overlay rendering to allow any number of active GL-GUI
   elements (like manipulators are now).
   New classes TGLOverlayElement.

8. Trailing white space has been removed in all files.


Restructure low-level interface to OpenGL (Timur)
=========================================

1. Allow sharing GL resources among viewers.

2. Do not bind GL-output (windowed or offscreen) to ROOT GUI.

New classes TGLContext, TGLFormat, TGLWidget.

This oboletes TGLKernel and TVirtualGL classes.


git-svn-id: http://root.cern.ch/svn/root/trunk@19000 27541ba8-7e3a-0410-8455-c3a389f83636