improved r_shadows mode, new cvars for better use with overhead camera mods:

improved r_shadows mode, new cvars for better use with overhead camera mods:
- r_shadows_darken : how much shadowed areas will be darkened (was constant 0.5)
- r_shadows_throwdirection : override shadow direction for r_shadows 2 (was constant 0 0 -1)
- r_shadows_drawafterrtlightning : this change fake shadows to draw AFTER realtime lightning is drawn, may be useful for simulating faster sunlight on large outdoor maps with only one noshadow rtlight. The price is less realistic appearance of dynamic light shadows.
- r_shadows_castfrombmodels : allow to cast fake shadows from bmodels

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@9201 d7cf8633-e32d-0410-b094-e92efae38249