Grid stripes aren't the full grid width when column count is specified

**Describe the bug**
`striped(true)` and `num_columns(N)` `Grid` paints only the minimum row width, though description of `num_columns` says that the last column will extend to fill the Ui

**To Reproduce**
for example,
```rust
ScrollArea::vertical()
.max_width(f32::INFINITY)
.auto_shrink([false, false])
.show(ui, |ui| {
egui::Grid::new("event_list_grid")
.striped(true)
.num_columns(4)
.show(ui, |ui| {
// draw some rows here
});
});
```

**Expected behavior**
It should fill entire row if `num_columns` is set

**Screenshots**
(Grid is directly in that ScrollArea, its max_rect is the rectangle of almost entire window)