LibGdx Game.setScreen appears to run extra update

- Still extending Game class
- Using StateManager.SwitchScreen instead of PGCGameCore.setScreen
- Added BaseScreen.update; each screen is calling it, but need to expose
private _isVisible from BasicScreen in GJLib

NOTE: passing deltaTime to update appears to make the update want to
"catch-up" - can reproduce by switching from main screen to any other
screen in debug mode (F2, F3, etc) and then back in a second (F1) -
observe the flying ship "catch up" to where it would have moved to if
the screen was updating all the time.