Add info_room and info_room_target

[khanghugo]

Two new entities, info_room and info_room_target. They both are meant to be used with one another.

They work like info_overview_point "reverse mode" where "everything outside the room can see everything inside the room". But this is a lot more customizable where we can select where "outside" is.

info_room indicates where you want to see entities inside info_room_target.

Here is an example map.
Archive.zip

I think there are a lot of use cases to this. Maybe a 3D skybox, multiple skyboxes, or very high resolution skybox (they all need to be a studio model). Those could be enclosed inside a visbreak box. info_room_target will be inside that box. Now we just need to scatter info_room appropriately so the skybox model would render. I think some other people might have better ideas than I do.