A set of n cloned tiles is counted only once toward the total cache
size, so each inidividual tile can be thought of as contributing
only 1/n of its size to the total cache size.  On the other hand,
the cost of stroing a tile is independent of the number of clones
it has.

When trimming the cache, remove an unstored tile with n cached
clones with a probability of 1/n, instead of 1, to avoid saturating
the swap queue with cloned tiles, causing stalls.