Graphic3d_Group is now abstract class and should be instantiated
using Graphic3d_Structure::NewGroup() method.
Graphic3d_CGroup has been removed.
The groups list is no more duplicated within Graphic3d_Structure and OpenGl_Structure.
Removed unused/duplicated fields from Graphic3d_Group.
Prs3d_Presentation - remove redundnant field myCurrentGroup.
PrsMgr_Presentation3d::Erase(), ::Clear() - avoid possible NULL dereference.
OpenGl_GraphicDriver/Graphic3d_GraphicDriver - group management methods have been removed
(moved to Graphic3d_CStructure and Graphic3d_Group interfaces).
Graphic3d_Group::GroupPrimitivesAspect() - copy IsEmission flag
Fix wrong iterator
Fix compilation issues
