LibWeb: Add <canvas> element and start fleshing out CRC2D

This patch adds HTMLCanvasElement along with a LayoutCanvas object.
The DOM and layout parts are very similar to <img> elements.

The <canvas> element holds a Gfx::Bitmap which is sized according to
the "width" and "height" attributes on the element.

Calling .getContext("2d") on a <canvas> element gives you a context
object that draws into the underlying Gfx::Bitmap of the <canvas>.
The context weakly points to the <canvas> which allows it to outlive
the canvas element if needed.

This is really quite cool. :^)

Libraries/LibWeb/DOM/CanvasRenderingContext2D.h

@@ -0,0 +1,38 @@
+#pragma once
+
+#include <AK/RefCounted.h>
+#include <LibGfx/Color.h>
+#include <LibGfx/Forward.h>
+#include <LibWeb/Bindings/Wrappable.h>
+
+namespace Web {
+
+class CanvasRenderingContext2D
+    : public RefCounted<CanvasRenderingContext2D>
+    , public Bindings::Wrappable {
+
+    AK_MAKE_NONCOPYABLE(CanvasRenderingContext2D);
+    AK_MAKE_NONMOVABLE(CanvasRenderingContext2D);
+
+public:
+    using WrapperType = Bindings::CanvasRenderingContext2DWrapper;
+
+    static NonnullRefPtr<CanvasRenderingContext2D> create(HTMLCanvasElement& element) { return adopt(*new CanvasRenderingContext2D(element)); }
+    ~CanvasRenderingContext2D();
+
+    void set_fill_style(String);
+    String fill_style() const;
+
+    void fill_rect(int x, int y, int width, int height);
+
+private:
+    explicit CanvasRenderingContext2D(HTMLCanvasElement&);
+
+    OwnPtr<Gfx::Painter> painter();
+
+    WeakPtr<HTMLCanvasElement> m_element;
+
+    Gfx::Color m_fill_style;
+};
+
+}