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/Bindings/NodeWrapper.cpp

@@ -26,12 +26,25 @@
 
 #include <LibJS/Runtime/PrimitiveString.h>
 #include <LibJS/Runtime/Value.h>
+#include <LibWeb/Bindings/DocumentWrapper.h>
+#include <LibWeb/Bindings/HTMLCanvasElementWrapper.h>
 #include <LibWeb/Bindings/NodeWrapper.h>
+#include <LibWeb/DOM/Document.h>
+#include <LibWeb/DOM/HTMLCanvasElement.h>
 #include <LibWeb/DOM/Node.h>
 
 namespace Web {
 namespace Bindings {
 
+NodeWrapper* wrap(JS::Heap& heap, Node& node)
+{
+    if (is<Document>(node))
+        return static_cast<NodeWrapper*>(wrap_impl(heap, to<Document>(node)));
+    if (is<HTMLCanvasElement>(node))
+        return static_cast<NodeWrapper*>(wrap_impl(heap, to<HTMLCanvasElement>(node)));
+    return static_cast<NodeWrapper*>(wrap_impl(heap, node));
+}
+
 NodeWrapper::NodeWrapper(Node& node)
     : EventTargetWrapper(node)
 {