Added gcc specific implementation of mul128 using __int128 intrinsic type.

Added unit tests for mul128 implementation.

Added r_shadow_bouncegrid_normalizevectors cvar, default on, this slightly brightens typical scenes, some more affected than others.

Changed the behavior of r_shadow_bouncegrid_dynamic_stablerandom to also affect static mode, changed it to use the light position as the RNG seed, and changed the meaning of stablerandom 1 to be lhcheeserandom again as it is fast and of sufficient quality, the Lehmer RNG is available as stablerandom 2 but it has a framerate hit in dynamic mode that I can't ignore.

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12301 d7cf8633-e32d-0410-b094-e92efae38249