implemented shadow caster culling by creating a frustum plane set that includes the...

implemented shadow caster culling by creating a frustum plane set that includes the light origin, any object that is not within this frustum (which encloses the screen and a triangular-shaped volume extending to the light origin) can not possibly cast a shadow onto the visible geometry, this reduces cpu work in a few cases (off-screen explosion with several entities further off-screen)

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@6945 d7cf8633-e32d-0410-b094-e92efae38249