We now support rAF, driven by GUI::DisplayLink callbacks. It's a bit
strange how we keep registering new callbacks over and over.
That's something we can definitely optimize.
This allows you to update animations/whatever without doing it more
often than the browser can display.
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. :^)
