Fix (ish) alloy behaviour

As far as I can tell, most alloys were being spawned from the wreckage
objects, not the ships themselves, and at some point in 1.77-1.80 ended
up properly inheriting their superheated temperature and exploding
instantly.

I've changed this around to give what seems to be roughly similar
behaviour to before - maybe 1 piece sometimes from light fighters,
probably 1, occasionally 2 from bigger ships. Wreckage now has its
fragment_chance set to 0, and a bug related to that setting is also
fixed.

Also increased the potential wreckage count from really big explosions,
if the graphics settings are high enough.