Depletion frame calculation

[hallipr]

You should be able to statically calculate, for a given set of food and creatures, spoil rate of the food and eating rate of the ceeatures.

This means a trough can have a function calculateFrame(time, food) that returns the the food with spoil and depletion rates, and creatures with food, rate, age.

The next frame time would be the min of any food depletion time or creature age up time.