LibHTML: Rewrite inline and text layout

Inline layout is now done by LayoutBlock. Blocks with inline children
will split them into line boxes during layout.

A LayoutBlock can have zero or more LineBox objects. Each LineBox
represents one visual line.

A LineBox can have any number of LineBoxFragment children. A fragment
is an offset+length into a specific LayoutNode.

To paint a LayoutBlock with inline children, we walk its line boxes,
and walk their fragments, painting each fragment at a time by calling
LineBoxFragment::render(), which in turn calls the LayoutNode via
LayoutText::render_fragment(). Hit testing works similarly.

This is very incomplete and has many bugs, but should make it easier
for us to move forward with this code.

Libraries/LibHTML/Layout/LineBoxFragment.h

@@ -0,0 +1,32 @@
+#pragma once
+
+#include <LibDraw/Rect.h>
+
+class LayoutNode;
+class RenderingContext;
+
+class LineBoxFragment {
+    friend class LineBox;
+public:
+    LineBoxFragment(const LayoutNode& layout_node, int start, int length, const Rect& rect)
+        : m_layout_node(layout_node)
+        , m_start(start)
+        , m_length(length)
+        , m_rect(rect)
+    {
+    }
+
+    const LayoutNode& layout_node() const { return m_layout_node; }
+    int start() const { return m_start; }
+    int length() const { return m_length; }
+    const Rect& rect() const { return m_rect; }
+    Rect& rect() { return m_rect; }
+
+    void render(RenderingContext&);
+
+private:
+    const LayoutNode& m_layout_node;
+    int m_start { 0 };
+    int m_length { 0 };
+    Rect m_rect;
+};