Using a function for sizing results in an incorrect total size once the items get all sized

[dtzxporter]

https://github.com/skayo/svelte-tiny-virtual-list/blob/1cc31b6b919b1624c24286445367ec8bc40727cf/src/SizeAndPositionManager.js#L230

This looks like part of the issue. (if you have 50 items, and they are all measured (last = 49), this will still add estimated to the final size.

May be other issues with the function, but it works properly just passing an array and recomputing the sizes as needed.