Make items_per_trigger = 0 a special case

[PennyJim]

Currently SpoiltoTrigger::items_per_trigger only allows positive values.
My hope is that setting it to 0 would allow us to indicate that we only want it to trigger at most once per stack.

I'm playing with the idea of a script trigger to allow a stack to very specifically decay (spoil) in half. Problem being, that with a very large stack, it'll trigger it so so many times... My method can't even do all that good of an early return to mitigate it.

[protocol_1903]

+1

[PennyJim]

Completed as of 2.0.42 with 127568

(and I object to the fact that this was poorly formulated.. I thought the multiple triggers was intentional)