layout: Properly parent table-row and table-row-group (#31619)

Put table cell content fragments into a hieararchy of fragments that
include their table row and table row group fragments. This ensures that
things like relative positioning and transforms set on rows and row
groups properly affect cells and cell content.

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

components/layout_2020/geom.rs

@@ -4,7 +4,7 @@
 
 use std::convert::From;
 use std::fmt;
-use std::ops::{Add, AddAssign, Sub};
+use std::ops::{Add, AddAssign, Sub, SubAssign};
 
 use app_units::Au;
 use serde::Serialize;
@@ -26,7 +26,7 @@ pub type LengthOrAuto = AutoOr<Length>;
 pub type AuOrAuto = AutoOr<Au>;
 pub type LengthPercentageOrAuto<'a> = AutoOr<&'a LengthPercentage>;
 
-#[derive(Clone, Serialize)]
+#[derive(Clone, Copy, Serialize)]
 pub struct LogicalVec2<T> {
     pub inline: T,
     pub block: T,
@@ -117,6 +117,34 @@ where
     }
 }
 
+impl<T> AddAssign<LogicalVec2<T>> for LogicalVec2<T>
+where
+    T: AddAssign<T> + Copy,
+{
+    fn add_assign(&mut self, other: LogicalVec2<T>) {
+        self.add_assign(&other);
+    }
+}
+
+impl<T> SubAssign<&'_ LogicalVec2<T>> for LogicalVec2<T>
+where
+    T: SubAssign<T> + Copy,
+{
+    fn sub_assign(&mut self, other: &'_ LogicalVec2<T>) {
+        self.inline -= other.inline;
+        self.block -= other.block;
+    }
+}
+
+impl<T> SubAssign<LogicalVec2<T>> for LogicalVec2<T>
+where
+    T: SubAssign<T> + Copy,
+{
+    fn sub_assign(&mut self, other: LogicalVec2<T>) {
+        self.sub_assign(&other);
+    }
+}
+
 impl<T: Zero> LogicalVec2<T> {
     pub fn zero() -> Self {
         Self {