Fix #1466 (Adds custom food filling check)

Custom foods that didn't keep their ingredients inside them weren't getting proper filling overlays.
Now their ```update()``` assigns them one with a random filling color if they don't have any already.
Fixes #1466 (Candy Maker producing invisible candy).

![candycolor](https://cloud.githubusercontent.com/assets/6355724/4263040/655e7020-3bc6-11e4-8884-828950a4f758.png)