reorganized sv.writeentitiestoclient handling of Mod_CanSeeBox, no

reorganized sv.writeentitiestoclient handling of Mod_CanSeeBox, no
longer tests player prediction for every entity, reworked to accommodate
support for portals and other remote cameras at some point in the future
added mod_q3bsp_tracelineofsight_brushes cvar (defaults to 0)
added sv_cullentities_trace_entityocclusion cvar (defaults to 0) which
enables a form of visibility culling even in entirely dynamic scenes
(for example instanced buildings)
these cvars allow much more aggressive culling at a higher cost

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@9340 d7cf8633-e32d-0410-b094-e92efae38249