Implement CEF3 and get basic debug information running

[axefrog]

CEF3 is cross-platform, supports offscreen rendering and can save huge amounts of time that would be wasted developing user interface code.

https://cefbuilds.com/
https://github.com/cefsharp/CefSharp
https://github.com/cefsharp/cef-binary (NuGet package should do everything)

This will require being able to render a texture to a polygon and composite that into the scene.

Task completion = CEF3 displaying a simple debug overlay showing mouse position and frame rate, and a verification that I can detect when input should go to the UI and when it should go directly to the game engine.