Support splitscreen in Camera system

[benreid24]

Unsure of the best way to do this for now. Currently there is a single stack of cameras and the engine sets the view before calling render on the active state.

Considerations:

• Render system should support splitscreen. Splitscreen doesn't really change render system per se, but it should be considered
• Pull letterboxing out from engine into ViewUtil, would be helpful for configuring splitscreen views into their regions
• The camera stack as it exists now may go away when the full render system is built. Thinking that the current scene could contain a set of active cameras. Cameras would include where to render to on the screen, thus supporting splitscreen. Would be helpful to maintain a normalized viewport for the usable rendering region so that top-level letterboxing continues to work