LibWeb: Bring CSS line-height implementation closer to spec

We now distribute the line-height evenly between the space above and
below inline-level boxes. This noticeably improves our baseline
alignment in many cases.

Note that the "vertical-align: <length>" case is quite awkward, as the
extra height added by the offset baseline must count towards the line
box height.

There's a lot of room for improvement here, but this makes the buckets
container on Acid3 show up in the right place, with the right size.

Userland/Libraries/LibWeb/Layout/LineBox.h

@@ -16,6 +16,7 @@ public:
     LineBox() = default;
 
     float width() const { return m_width; }
+    float height() const { return m_height; }
     float bottom() const { return m_bottom; }
     float baseline() const { return m_baseline; }
 
@@ -36,6 +37,7 @@ private:
 
     Vector<LineBoxFragment> m_fragments;
     float m_width { 0 };
+    float m_height { 0 };
     float m_bottom { 0 };
     float m_baseline { 0 };
 };