Bolts and bullets should fade out rather than be culled

[olofson]

As it is, both player bolts and enemy bullets are simply removed when they get off-screen, which doesn't make all that much sense from a game play point of view. This also makes the game heavily dependent on camera movement (already increased player bolt lifetime to deal with the new smooth camera), and it would cause even more weirdness with multiplayer.

Bolts and bullets should just "burn out" and vanish over a certain time or distance, regardless of who or what may be observing them.

[olofson]

See also #98; there are bugs that would be eliminated in a simple and robust way by this feature.