Render portals only when necessary

[bliptec]

Portals don't need to render when they're completely obstructed by geometry or outside the player's view.

[MattJeanes]

Props by default don't render when the player is not looking at them, would it be possible to use the same method for this or is it not exposed to Lua? Other than that, I can only think of using the dot product but that's fairly expensive iirc, and it wouldn't work for portals behind geometry. I suppose you could do a trace of some sort for that but again that would be fairly expensive.

[bliptec]

as of 434c695 the portal is drawn like a normal entity, but the view is still rendered. It'd be nice to check if the player is behind the portal too, since the math for that is significantly less expensive than re-rendering the world.