layout: Start work on table row height and vertical-align (#31246)

This implements a very naive row height allocation approach. It has just
enough to implement `vertical-align` in table cells. Rowspanned cells
get enough space for their content, with the extra space necessary being
allocated to the last row. There's still a lot missing here, including
proper distribution of row height to rowspanned cells.

Co-authored-by: Oriol Brufau <obrufau@igalia.com>

components/layout_2020/fragment_tree/box_fragment.rs

@@ -202,6 +202,7 @@ impl BoxFragment {
                 \nmargin={:?}\
                 \nclearance={:?}\
                 \nscrollable_overflow={:?}\
+                \nbaselines={:?}\
                 \noverflow={:?} / {:?}",
             self.base,
             self.content_rect,
@@ -210,6 +211,7 @@ impl BoxFragment {
             self.margin,
             self.clearance,
             self.scrollable_overflow(&PhysicalRect::zero()),
+            self.baselines,
             self.style.get_box().overflow_x,
             self.style.get_box().overflow_y,
         ));