[ optim ] The exported image can be frozen in an unsafe manner

Given that the Mutable version will not be altered later, we can
simply freeze it in an unsafe manner rather than having to pay
an additional O(n) cost.
[ fix ] definition of 'crop'

Yesterday's commit was buggy: when inserting a grid in a subpainting,
one may very well create something too big to fit in its cell. `crop`
comes to the rescue & cuts everything up to the right size.

We can now generate big images without having out of bound errors.