Added proper cleaning of LOD groups:

Added proper cleaning of LOD groups:
- Graphic3d_Structure's Remove and GraphicClear now take into account LOD structures;
- LOD graphic resources are cleaned up properly in OpenGl_Structure;
Correction of warnings.

TODO:
There are a lot of places in OpenGl_Structure where the structure tries to iterate through its own graphic groups.
In case of objects represented as LODs, such loops are useless and likely will affect the performance.