Overhauled r_shadow_bouncegrid, it performs much faster, makes use of as many threads...

Overhauled r_shadow_bouncegrid, it performs much faster, makes use of as many threads as you wish, and has a nicer calculation for light path accumulation.

r_shadow_bouncegrid now does refractive index calculations on water surfaces, it's not very obvious yet because the traceline doesn't return texture information (if it did, there would be cool caustics effects in dynamic mode).

Implemented TaskQueue system for simple threaded tasks (fibers but less featureful), this is used by r_shadow_bouncegrid but will be used by other parts of the engine in future, also added Thread_Atomic* functions.

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12493 d7cf8633-e32d-0410-b094-e92efae38249