Changed tile probability does not affect brush until the stamp is changed

[bjorn]

There are a few occasions upon which the RandomPicker<Cell> used by the stamp brush is updated, but changing the probability of a tile is not one of them.

Of course, since it would be silly to update it each time a probability changes, the change should only invalidate this "cache" such that it is updated when the stamp brush is used again.

Problem reported on the forum.