Optimise render time for complex scenes by using an R*-Tree

I have used an implementation of R*-Tree from [here](http://www.virtualroadside.com/blog/index.php/2008/10/04/r-tree-implementation-for-cpp/). The optimisation results in roughly ten times faster render time for complex scenes (e.g. the ones found in Test/Suites/SPD), while offers no performance improvement for simpler scenes.