From a bug report, pets able to eat
acidic and poisonous corpses (black naga was the case cited) would eat
green slime corpses without turning into green slime, unlike the hero.
This prevents such monsters from eating green slime unless they're
starving, implements transformation into green slime for the case where
it does get eaten, and prevents non-pet gelatinous cubes from devouring
such corpses. meatobj() is reorganized to hopefully become clearer, and
it removes the assumption that the object eater is a g.cube in case we
ever adopt slash'em's "tasmanian devil" monster.
Monsters with digestion attacks who swallow green slime monsters
are turned into green slime, but ones who swallow hero poly'd into green
slime are not. This doesn't address that.