changed use of infinite farclip to depend on 32bpp modes instead of on

changed use of infinite farclip to depend on 32bpp modes instead of on
whether shadows are enabled
added r_useinfinitefarclip cvar to allow disabling this otherwise forced
feature if it causes problems for anyone (was on for most users already)
optimized farclip math a little bit (use model->radius * 2 instead of
VectorDistance from corner to corner)

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@8215 d7cf8633-e32d-0410-b094-e92efae38249