when erasing the display ensure line size matches physical

In the case where the screen was enlarged, we don't eagerly resize
the line vectors.

If a subsequent erase display occurred, we'd only erase to the old
size of the lines, rather than the new size, which could result
in a black square in the upper right corner of the display.

Repro scenario is:

* resize window larger than initial height
* `dialog --yesno hello 0 0`

The dialog background should fill the display.

term/src/screen.rs

@@ -130,8 +130,10 @@ impl Screen {
         cols: impl Iterator<Item = usize>,
         attr: &CellAttributes,
     ) {
+        let physical_cols = self.physical_cols;
         let line_idx = self.phys_row(y);
         let line = self.line_mut(line_idx);
+        line.resize(physical_cols);
         line.fill_range(cols, &Cell::new(' ', attr.clone()));
     }