Let monsters cast prisms non-adjacent to their foe.
Don't try to fire a tracer for Fulminant Prism.

It doesn't work unless the caster and target and chosen position are lined
up just right, since the tracer doesn't explode.

There does need to be some kind of check to avoid hurting allies (and
oneself), but probably _mons_prism_pos would be the place to do that.