Extend the TVirtualGL interface to support 16 & 32 bit display mode.
This is required for the new slicing algorithm (eg to run the tutorial
glbox.C under Windows in 16 bits display mode)


git-svn-id: http://root.cern.ch/svn/root/trunk@16607 27541ba8-7e3a-0410-8455-c3a389f83636