LibWeb: Support <svg> elements with display: block

There are a couple of things that went into this:

- We now calculate the intrinsic width/height and aspect ratio of <svg>
  elements based on the spec algorithm instead of our previous ad-hoc
  guesswork solution.

- Replaced elements with automatic size and intrinsic aspect ratio but
  no intrinsic dimensions are now sized with the stretch-fit width
  formula.

- We take care to assign both used width and used height to <svg>
  elements before running their SVG formatting contexts. This ensures
  that the inside SVG content is laid out with knowledge of its
  viewport geometry.

- We avoid infinite recursion in tentative_height_for_replaced_element()
  by using the already-calculated used width instead of calling the
  function that calculates the used width (since that may call us right
  back again).

Userland/Libraries/LibWeb/Layout/SVGSVGBox.h

@@ -19,6 +19,7 @@ public:
     virtual ~SVGSVGBox() override = default;
 
     SVG::SVGSVGElement& dom_node() { return verify_cast<SVG::SVGSVGElement>(ReplacedBox::dom_node()); }
+    SVG::SVGSVGElement const& dom_node() const { return verify_cast<SVG::SVGSVGElement>(ReplacedBox::dom_node()); }
 
     virtual bool can_have_children() const override { return true; }
 
@@ -28,6 +29,8 @@ public:
 
 private:
     virtual bool is_svg_svg_box() const final { return true; }
+
+    [[nodiscard]] Optional<float> calculate_intrinsic_aspect_ratio() const;
 };
 
 template<>