layout: Add support for table rows, columns, rowgroups and colgroups (#31341)

This adds support for table rows, columns, rowgroups and colgroups.
There are few additions here:

1. The createion of fragments, which allows script queries and hit
   testing to work properly. These fragments are empty as all cells are
   still direct descendants of the table fragment.
2. Properly handling size information from tracks and track groups as
   well as frustrating rules about reordering rowgroups.
3. Painting a background seemlessly across track groups and groups. This
   is a thing that isn't done in legacy layout (nor WebKit)!

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

components/layout_2020/fragment_tree/box_fragment.rs

@@ -2,6 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
+use app_units::Au;
 use gfx_traits::print_tree::PrintTree;
 use serde::Serialize;
 use servo_arc::Arc as ServoArc;
@@ -20,6 +21,11 @@ use crate::geom::{
 };
 use crate::style_ext::ComputedValuesExt;
 
+pub(crate) struct ExtraBackground {
+    pub style: ServoArc<ComputedValues>,
+    pub rect: LogicalRect<Au>,
+}
+
 #[derive(Serialize)]
 pub(crate) struct BoxFragment {
     pub base: BaseFragment,
@@ -62,6 +68,9 @@ pub(crate) struct BoxFragment {
     /// during stacking context tree construction because they rely on the size of the
     /// scroll container.
     pub(crate) resolved_sticky_insets: Option<PhysicalSides<LengthOrAuto>>,
+
+    #[serde(skip_serializing)]
+    pub extra_backgrounds: Vec<ExtraBackground>,
 }
 
 impl BoxFragment {
@@ -149,6 +158,7 @@ impl BoxFragment {
             scrollable_overflow_from_children,
             overconstrained,
             resolved_sticky_insets: None,
+            extra_backgrounds: Vec::new(),
         }
     }
 
@@ -165,6 +175,10 @@ impl BoxFragment {
         self
     }
 
+    pub fn add_extra_background(&mut self, extra_background: ExtraBackground) {
+        self.extra_backgrounds.push(extra_background);
+    }
+
     pub fn scrollable_overflow(
         &self,
         containing_block: &PhysicalRect<Length>,