Duke3D: Re-enable remote activation of Viewscreen using yvel of 1

This makes it so that a viewscreen placed in the map with a yvel of 1 will always display its camera feed if the player approaches, which replicates DOS behavior.

If the screen is connected to a camera, it will display the view of that camera. Otherwise, it will show the view of the Viewscreen's owner, the direction in which it is looking being dictated by angle and shade of that sprite. Owner is by default the viewscreen itself.

Additionally, the xvel of the viewscreen now defines the active range of the screen. If set to 0, uses default hardcoded range of 8192.

Note that this is currently restricted to one active screen at a time.

Partially resolves #41 (closed)