Fix grid scroll position resetting to 0
Fix grid scroll position resetting to 0

The problem was that the index of the scrollbars was not being updated.
Therefore, it was always being reset to 0 each time the animation frame
is requested.

Fixed this by updating the index of the scrollbars. This also eliminates
the need to call the grid's scrollBy method.

fin-hypergrid/core,fin-hypergrid/core