Fix dynamic start with offset

[runspired]

This adds a (was failing) test for starting a dynamically sized list in a position offset from the top of the scroll-container.

However, this causes the skip-list to need to deal with negative numbers. To handle this, I then changed the determination of middleVisibleValue to:

let top = this.visibleTop;

if (top < 0) {
  top = 0;
}

const middleVisibleValue = top + (this.scrollContainerHeight / 2);

[pzuraq]

Yeah, we should keep visibleTop here. scrollTopOffset is still needed, and prependOffset is whatever the offset we get when we do a prepend (in a prepend, we don't actually set scrollTop until after we've rendered to avoid setting it more than once, so we need to make our calculations with that in mind).

The correct behavior here is if visibleTop <= 0, we would render the first n elements (where n is the number of VCs). visibleTop is supposed to give us the ability to have a "negative scrollTop", which is useful for calculations and tracking state but makes no sense here. So it should probably be:

const middleVisibleValue = Math.max(this.visibleTop, 0) + (this.scrollContainerHeight / 2);

[pzuraq]

Looks good in tests pass