LibWeb: Create "empty" line box fragments for inline elements

In order for inline elements (e.g <span>) to contribute padding etc.
to line boxes, we now create special "leading" and "trailing" fragments
for Layout::InlineNode and size them according to the horizontal
padding values.

The height of these fragments is taken from the tallest fragment on the
line. (Perhaps we should stop having per-fragment heights and just keep
a single height per line box, but that's a separate issue.)

In order to make things look nice, we now also adjust the height of all
fragments on a line so that nobody is shorter than the CSS line-height.

Libraries/LibWeb/Layout/InlineNode.h

@@ -36,6 +36,10 @@ public:
     virtual ~InlineNode() override;
     virtual const char* class_name() const override { return "InlineNode"; }
 
+    void paint_fragment(PaintContext&, const LineBoxFragment&, PaintPhase) const;
+
+    virtual void split_into_lines(BlockBox& containing_block, LayoutMode) override;
+
 private:
     virtual bool is_inline_node() const final { return true; }
 };