Reverse mapping of light pointers

[wahn]

After the book was published they added reverse mapping for light pointers:

    // Compute a reverse mapping from light pointers to offsets into the                                
    // scene lights vector (and, equivalently, offsets into                                             
    // lightDistr). Added after book text was finalized; this is critical                               
    // to reasonable performance with 100s+ of light sources.                                           

This was done for the BDPTIntegrator as well as for the MLTIntegrator. So far we did not optimize this because we didn't render scenes with that many lights and the algorithms work without this optimization.

[wahn]

Closing for now ... maybe it should be re-opened in the future ...