Fix buggy selection when scrolling down

When scrolling down with a selection on screen the first line was not
properly selected. This has been fixed by making sure the selection
always starts in the first cell when it is only partially visible.

src/term/mod.rs

@@ -134,7 +134,7 @@ impl<'a> RenderableCellsIter<'a> {
                     Some((start_line, loc.start.col, end_line, loc.end.col))
                 },
                 (Some(start_line), None) => {
-                    Some((start_line, loc.start.col, Line(0), grid.num_cols()))
+                    Some((start_line, loc.start.col, Line(0), Column(0)))
                 },
                 (None, Some(end_line)) => {
                     Some((grid.num_lines(), Column(0), end_line, loc.end.col))