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/LineBoxFragment.h

@@ -37,12 +37,19 @@ class LineBoxFragment : public Weakable<LineBoxFragment> {
     friend class LineBox;
 
 public:
-    LineBoxFragment(const Node& layout_node, int start, int length, const Gfx::FloatPoint& offset, const Gfx::FloatSize& size)
+    enum class Type {
+        Normal,
+        Leading,
+        Trailing,
+    };
+
+    LineBoxFragment(const Node& layout_node, int start, int length, const Gfx::FloatPoint& offset, const Gfx::FloatSize& size, Type type)
         : m_layout_node(layout_node)
         , m_start(start)
         , m_length(length)
         , m_offset(offset)
         , m_size(size)
+        , m_type(type)
     {
     }
 
@@ -50,12 +57,14 @@ public:
     int start() const { return m_start; }
     int length() const { return m_length; }
     const Gfx::FloatRect absolute_rect() const;
+    Type type() const { return m_type; }
 
     const Gfx::FloatPoint& offset() const { return m_offset; }
     void set_offset(const Gfx::FloatPoint& offset) { m_offset = offset; }
 
     const Gfx::FloatSize& size() const { return m_size; }
     void set_width(float width) { m_size.set_width(width); }
+    void set_height(float height) { m_size.set_height(height); }
     float width() const { return m_size.width(); }
     float height() const { return m_size.height(); }
 
@@ -77,6 +86,7 @@ private:
     int m_length { 0 };
     Gfx::FloatPoint m_offset;
     Gfx::FloatSize m_size;
+    Type m_type { Type::Normal };
 };
 
 }