added r_shadow_bouncegrid_static cvar which makes a proper static mode

added r_shadow_bouncegrid_static cvar which makes a proper static mode
and this is now the default, its framerate cost is very low and it makes
realtime world lighting look nicer, tweaked all relevant cvars for this
detect whether any lights warrant bouncegrid each frame - this means
that the cvar could be defaulted on and still do nothing when realtime
world lighting is turned off

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@10971 d7cf8633-e32d-0410-b094-e92efae38249