LibWeb: Align actual baselines, not just bottoms

Until now, we've been treating the bottom of every line box fragment as
its baseline, and just aligning all the bottoms to the bottom of the
line box. That gave decent results in many cases, but was not correct.

This patch starts moving towards actual baseline calculations as
specified by CSS2.

Note that once layout is finished with a line box, we also store the
baseline of the line box in LineBox::m_baseline. This allows us to align
the real baseline of display:inline-block elements with other inline
content on the same line.

Userland/Libraries/LibWeb/Layout/LineBox.h

@@ -17,6 +17,7 @@ public:
 
     float width() const { return m_width; }
     float bottom() const { return m_bottom; }
+    float baseline() const { return m_baseline; }
 
     void add_fragment(Node const& layout_node, int start, int length, float leading_size, float trailing_size, float content_width, float content_height, float border_box_top, float border_box_bottom, LineBoxFragment::Type = LineBoxFragment::Type::Normal);
 
@@ -36,6 +37,7 @@ private:
     Vector<LineBoxFragment> m_fragments;
     float m_width { 0 };
     float m_bottom { 0 };
+    float m_baseline { 0 };
 };
 
 }