Request - multiple items per row (grid system)

**I'm submitting a feature request**
We want to use ui-virtualization with an grid system. At this moment it works great with items that are full width and have an fixed height, but fails when we use a system like 'Material Design Components' or 'Bootstrap'.

We have an page with a large array of components that are aligned with 3 items in 1 row and on smaller screen this goes to 2 items and eventually to just 1.

**Current behavior:**
UI-Virtualization puts a emtpy item infront of the first item. This adjusts the grid so the very first item isn't aligned properly. After that all the items are not in their correct position.

**Expected/desired behavior:**
It is disered that UI-Virtualization works with a variable grid system that can have multiple items per row.

Reproduce the reported behavior with ui-virtualization and a responsive grid containing three, two, and one item per row. Investigate how the current virtualization handles full-width items and fixed heights; done means variable row layouts render without an empty leading item and remain correctly positioned across screen sizes.

Written by the indexing model from the issue text.