Add triangle backface culling outside of playable area

Consider playable areas to be parts of the map, containing player spawn and teleport target entities, enclosed by clip brushes(and maybe kill trigers). Any geometry not visible from inside of such area would not be visible during normal gameplay, but may still waste GPU resources(drawcall, vertex shader invocations, triangle setup, potential state changes) when visleaf containing it is visible. While map creators do often optimize maps by replacing texture on such faces with caulk, which requires a lot of manual labour, in some places it is not possible as it changes how light is reflected.