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/table/mod.rs

@@ -37,6 +37,9 @@ pub struct Table {
 
     /// The size of this table.
     pub size: TableSize,
+
+    /// Whether or not this Table is anonymous.
+    anonymous: bool,
 }
 
 impl Table {
@@ -45,6 +48,7 @@ impl Table {
             style,
             slots: Vec::new(),
             size: TableSize::zero(),
+            anonymous: false,
         }
     }