Place floats in layout 2020, but don't flow text around the floats yet.

This commit puts floats behind the `layout.floats.enabled` pref, because of the
following issues and unimplemented features:

* Inline formatting contexts don't take floats into account, so text doesn't
  flow around the floats yet.

* Non-floated block formatting contexts don't take floats into account, so BFCs
  can overlap floats.

* Block formatting contexts that contain floats don't expand vertically to
  contain all the floats. That is, floats can stick out the bottom of BFCs,
  contra spec.

components/layout_2020/geom.rs

@@ -251,6 +251,16 @@ impl<T> flow_relative::Sides<T> {
         self.block_start + self.block_end
     }
 
+    pub fn sum(&self) -> flow_relative::Vec2<T::Output>
+    where
+        T: Add + Copy,
+    {
+        flow_relative::Vec2 {
+            inline: self.inline_sum(),
+            block: self.block_sum(),
+        }
+    }
+
     pub fn to_physical(&self, mode: WritingMode) -> PhysicalSides<T>
     where
         T: Clone,
@@ -290,6 +300,18 @@ impl<T> flow_relative::Sides<T> {
     }
 }
 
+impl<T> flow_relative::Sides<T>
+where
+    T: Copy,
+{
+    pub fn start_offset(&self) -> flow_relative::Vec2<T> {
+        flow_relative::Vec2 {
+            inline: self.inline_start,
+            block: self.block_start,
+        }
+    }
+}
+
 impl flow_relative::Sides<&'_ LengthPercentage> {
     pub fn percentages_relative_to(&self, basis: Length) -> flow_relative::Sides<Length> {
         self.map(|s| s.percentage_relative_to(basis))