Add (optional) automatic entity killing when outside scene boundaries

[ensisoft]

Add (optional) automatic entity killing when outside scene boundaries.
There are 2 ways to kill entities that fall outside of game play (imagine for example a falling body that falls through the scene and no longer contributes to the game play). Either the game makes sure that the entity is killed when some criteria is met such as falling to a distance of X meters from the "baseline" in game. But this can be tedious and error prone depending on the exact use case.

Alternative would be to have the game (scene) define a "hard" boundary and any entity completely outside these bounds would then be automatically killed.

Obvious reasons for this killing are

• performance, get rid of entities that no longer contribute to the game play
• stability of the game in for example physics simulation, at some point the floats and the physics sim would blow up potentially creating unexpected events in the game.