Fix mass gain/loss being wrong when grown/shrunk

Changing a creature's mass sets its base mass so that, after
effects are applied, its mass is correct.

src/game/creature.ts

@@ -69,8 +69,12 @@ export class Creature extends Mortal {
         const adjusted = self.effects.reduce((scale: number, effect: Effective) => effect.scale(scale), base)
         return adjusted
       },
+      // we want to account for anything changing our current size;
+      // we will assume that the modifiers are all multiplicative
       set [VoreStat.Mass] (mass: number) {
-        self.baseMass = mass
+        const modifier = self.effects.reduce((scale: number, effect: Effective) => effect.scale(scale), 1)
+        const adjusted = mass / modifier
+        self.baseMass = adjusted
       },
       get [VoreStat.PreyCount] () {
         return self.containers.reduce(