Re-Add Fade Transitions

[Mako88]

This has been something I've missed since I first discovered Rebirth. In DOS you had a nice fade in/out between each screen change.

This should be toggleable because some may prefer the increased speed of not having to wait for the fading.

[ryusei117]

Ah yes, I miss those too. Somehow they manage to make the game a bit more immersive. :-) A similar feature that's also missing is the screen flash effect for the Cloaking Device: in the original engine, when you acquire a Cloak, the screen darkens briefly.