[Opensg-core] RFC/PATCH: Create RenderTreeNodes from TreeBuilders

[Carsten N. <car...@gm...>]

	Hello all,

attached patch series changes the way we create RenderTreeNodes from 
doing it inside RenderPartition::dropFunctor to TreeBuilderBase::add (or 
derived).
This seems a cleaner interface as the RPart now does not have to care 
what the different TreeBuilder actually store in the RTNode. It also 
allows moving occlusion culling specific data into OCRenderTreeNode that 
is only used by the OcclusionCullingTreeBuilder, reducing the size of 
the base RTNode.

Comments? [1]

	Cheers,
		Carsten

[1] I'll commit this at the end of the week if nobody objects.