Bg override for columns does not properly fill off-screen blank cells

Steps to reproduce:
1. Set the table height to 100%.
2. Ensure the number of rows is fewer than the available height (so no vertical scroll is rendered).
3. Override `bgCell` for the columns (see [this commit](https://github.com/mir4a/glide-data-grid/pull/1/commits/bae84b9233df404023a337a3b8581cd152d953ec) for the reference)
4. Ensure there is a horizontal scrollbar.
5. Scroll left-right

Screenshots:
1. The first render is correct. You can see that the cells in the blank rows are painted with the custom color as expected:

2. However, after scrolling left and right, the off-screen area has a painting issue: