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.cpp

@@ -0,0 +1,44 @@
+#include <AK/OwnPtr.h>
+#include <LibGfx/Painter.h>
+#include <LibWeb/DOM/CanvasRenderingContext2D.h>
+#include <LibWeb/DOM/HTMLCanvasElement.h>
+
+namespace Web {
+
+CanvasRenderingContext2D::CanvasRenderingContext2D(HTMLCanvasElement& element)
+    : m_element(element.make_weak_ptr())
+{
+}
+
+CanvasRenderingContext2D::~CanvasRenderingContext2D()
+{
+}
+
+void CanvasRenderingContext2D::set_fill_style(String style)
+{
+    m_fill_style = Gfx::Color::from_string(style).value_or(Color::Black);
+}
+
+String CanvasRenderingContext2D::fill_style() const
+{
+    return m_fill_style.to_string();
+}
+
+void CanvasRenderingContext2D::fill_rect(int x, int y, int width, int height)
+{
+    auto painter = this->painter();
+    if (!painter)
+        return;
+
+    painter->fill_rect({ x, y, width, height }, m_fill_style);
+}
+
+OwnPtr<Gfx::Painter> CanvasRenderingContext2D::painter()
+{
+    if (!m_element)
+        return nullptr;
+
+    return make<Gfx::Painter>(m_element->ensure_bitmap());
+}
+
+}