Fix rendering of selections outside the viewport

When rendering selections with both start and end outside of the visible
area, Alacritty would assume that both start and end are either above or
below the viewport and not render the selection at all.

To fix this the `buffer_line_to_visible` method now returns a
`ViewportPosition` instead of an `Option<Line>`, this allows giving more
feedback about where outside of the visible region the line is using the
`ViewportPosition::Above` and `ViewportPosition::Below` variants.

Using these newly introduced variants, a selection spanning the whole
screen is now rendered if the selection should go from above the visible
area to below it.

This fixes #1557.

0 files changed, 0 insertions, 0 deletions