layout: Hide collapsed borders crossed by spanning cells (#35129)

For example, a cell with `rowspan="2"` can cross a collapsed border that
was set on the rows. Now the slice of this row border that is crossed
by the cell will be hidden.

Signed-off-by: Oriol Brufau <obrufau@igalia.com>

components/layout_2020/style_ext.rs

@@ -240,6 +240,10 @@ impl BorderStyleColor {
             Self::new(border.border_left_style, border.border_left_color.clone()),
         )
     }
+
+    pub(crate) fn hidden() -> Self {
+        Self::new(BorderStyle::Hidden, Color::TRANSPARENT_BLACK)
+    }
 }
 
 impl Default for BorderStyleColor {