Ray-tracing based lighting mode

[VReaperV]

Exact form to be determined. It is likely that a lot of the computations can be cached, so a mix between ray-tracing/path-tracing and final gathering might work well.

The initial release of this feature will be aimed at mappers, to allow fast iteration of various lighting settings (simply because making it work fast enough to be used during a game will take more time and effort).