Removed unnecessary computation of base OpenGl_Structure for main presentation;

Removed unnecessary computation of base OpenGl_Structure for main presentation;
Bounding box of LOD Graphic3d_Structure is computed as combined box for all LOD graphic groups;
Method HasLevelsOfDetail() was added to presentable objects to mark objects presented via LODs at the time of Compute() call;
Redefineable method ComputeLods() was added to presentable object for customization.