cl-cairo2 in cl-gtk2

I've added Cairo graphics library support in cl-gtk2. I've gone the easy way by integrating with cl-cairo2.

The main use case for Cairo in Gtk+ is widget custom drawing. For this, in the expose-event handler, the Cairo context is created on which the drawing is performed.

To create a drawing context a cl-gtk2-cairo:with-gdk-context macro is used which creates and releases the Cairo context. Widget size is obtained with the gdk:drawable-get-size function that returns the width and the height as multiple return values.

As a result, the expose-event handler looks something like this:

(lambda (widget event)
  (declare (ignore event))
  (multiple-value-bind (w h) (gdk:drawable-get-size (widget-window widget))
    (with-gdk-context (ctx (widget-window widget))
      (with-context (ctx)
        (rectangle 0 0 w h)
        (set-source-rgb 1 1 1)
        (fill-path)
        nil))))

And here's a screenshot for the cl-gtk2-cairo-demo:demo: