LibWeb: Support inline-level padding and border properly

Here's roughly how this works:

- InlineLevelIterator keeps a nesting stack of inline-level nodes with
  box model metrics.
- When entering a node with box model metrics, we add them to the
  current "leading metrics".
- When exiting a node with box model metrics, we add them to the
  current "trailing metrics".
- Pending leading metrics are consumed by the first fragment added
  to the line.
- Pending trailing metrics are consumed by the last fragment added
  to the line.

Like before, the position of a line box fragment is the top left of its
content box. However, fragments are placed horizontally along the line
with space inserted for padding and border.

InlineNode::paint() now expands the content rect as appropriate when
painting background and borders.

Note that margins and margin collapsing is not yet implemented.

This makes the eyes on ACID2 horizontally centered. :^)

Userland/Libraries/LibWeb/Layout/LineBuilder.h

@@ -19,8 +19,8 @@ public:
     ~LineBuilder();
 
     void break_line();
-    void append_box(Box&);
-    void append_text_chunk(TextNode&, size_t offset_in_node, size_t length_in_node, float width, float height);
+    void append_box(Box&, float leading_size, float trailing_size);
+    void append_text_chunk(TextNode&, size_t offset_in_node, size_t length_in_node, float leading_size, float trailing_size, float content_width, float content_height);
 
     void break_if_needed(LayoutMode layout_mode, float next_item_width, bool should_force_break)
     {