I've finally took on Nadro's code from the forum and adapted it to overcome some restrictions of the original code. Moreover, there are some additional tweaks to make it simpler to use. A few minor things might still change in the future, but for now it looks like a proper implementation with many useful use cases. I'll come back on this in the next time when I provide an example for efficient usage of occlusion queries.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@3265 dfc29bdd-3216-0410-991c-e03cc46cb475