Vehicle cameras mod

The feature I've seen requested few times.

Cameras use mirror mechanics and are similar to them in many ways.

When sitting on a vehicle on a part flagged as "CAM_CONTROL", cameras work like mirrors seen by the player, except with offset being dependent on camera stats rather than distance to it.

I reuse VPFLAG_MIRROR, because it is used only in lightmap.cpp and only in this exact function that now also handles cameras. That way processing seen cache shouldn't be any slower when no mirrors or cameras are installed on current vehicle and only an incredibly tiny bit slower when there are many mirrors installed.

Cameras are installed on the roof, but (like turrets and overhead lights) they can't see through vehicle walls.