Fix severe bugs in mul128 implementation which was breaking the Lehmer RNG.

Fix severe bugs in mul128 implementation which was breaking the Lehmer RNG.

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