Add stateful version of `getter` dimensions based on accumulating state

Most use cases of grids with complex items want an accumulating state + wrapping etc. We ought to provide a helper for this, since the performance gains of not doing so (avoiding O(n) canvas size fetch) are moved onto the user. The docs would have sufficient warnings of the performance implications.

There is also a special case of this where the data you are providing is append-only. If we are informed this is the case, then the array changing makes the calculations O(number of elements added) instead of O(number of elements in the data source).