* TGLFormat, TGLWidget

  Add handling of multi-sampling formats for GLX.
  WGL implementation is still missing.
  The value is taken from TEnv "OpenGL.Framebuffer.Multisample".

* TGLViewerBase
  Use TGLCapabilityEnabler for switching on bleniding.

* TGLViewer
  Remove flas fInitGL and associated functions.
  Set sane GL state on every Draw().
  Consolidate image extraction from frame-buffers.

* TGLEmbeddedViewer, TGLSAViewer
  Remove calls to ResetInitGL().

* TGLRnrCtx
  Remove fGrabBuffer and fGrabbedImage members, only keep fGrabImage flag.
  All the rest is handled internally in TGLViewer now.

* TGLFBO
  Add support for multi-sampling FBOs.


git-svn-id: http://root.cern.ch/svn/root/trunk@36675 27541ba8-7e3a-0410-8455-c3a389f83636