LibWeb: Add inline-level iterator that enumerates items for line layout

This patch adds a new mechanism that allows InlineFormattingContext to
build line boxes incrementally instead of all-in-one go.

Incremental build will eventually allow much better support for CSS
floating objects.

Userland/Libraries/LibWeb/Layout/InlineLevelIterator.h

@@ -0,0 +1,64 @@
+/*
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Noncopyable.h>
+#include <LibWeb/Layout/Box.h>
+#include <LibWeb/Layout/TextNode.h>
+
+namespace Web::Layout {
+
+// This class iterates over all the inline-level objects within an inline formatting context.
+// By repeatedly calling next() with the remaining available width on the current line,
+// it returns an "Item" representing the next piece of inline-level content to be placed on the line.
+class InlineLevelIterator {
+    AK_MAKE_NONCOPYABLE(InlineLevelIterator);
+    AK_MAKE_NONMOVABLE(InlineLevelIterator);
+
+public:
+    struct Item {
+        enum class Type {
+            Text,
+            Element,
+            ForcedBreak,
+        };
+        Type type {};
+        Layout::Node* node { nullptr };
+        size_t offset_in_node { 0 };
+        size_t length_in_node { 0 };
+        float width { 0.0f };
+    };
+
+    explicit InlineLevelIterator(Layout::Box& container, LayoutMode layout_mode)
+        : m_container(container)
+        , m_current_node(container.first_child())
+        , m_layout_mode(layout_mode)
+    {
+    }
+
+    Optional<Item> next(float available_width);
+
+private:
+    void skip_to_next();
+
+    void enter_text_node(Layout::TextNode&);
+
+    Layout::Box& m_container;
+    Layout::Node* m_current_node { nullptr };
+    LayoutMode const m_layout_mode;
+
+    struct TextNodeContext {
+        bool do_collapse {};
+        bool do_wrap_lines {};
+        bool do_respect_linebreaks {};
+        TextNode::ChunkIterator chunk_iterator;
+    };
+
+    Optional<TextNodeContext> m_text_node_context;
+};
+
+}