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/BlockFormattingContext.cpp

@@ -577,6 +577,9 @@ void BlockFormattingContext::layout_block_level_box(Box const& box, BlockContain
 
     place_block_level_element_in_normal_flow_horizontally(box, available_space);
 
+    if (box.is_replaced_box())
+        compute_height(box, available_space);
+
     if (independent_formatting_context) {
         // This box establishes a new formatting context. Pass control to it.
         independent_formatting_context->run(box, layout_mode, box_state.available_inner_space_or_constraints_from(available_space));