Simulates alpha color support by rendering the Gdk window
content behind a popover on its background, if the Gdk
screen does not support alpha colors or window opacity.

* Use gdk_cairo_set_source_window() if possible (Gtk+ 2.24+)
* Optimize window to pixbuf rendering with Gtk+ < 2.24

On Windows and Mac gdk_cairo_set_source_window()
does not work properly.

If the platform has no alpha support and alpha
can not be faked, just fill the whole popover
with the desired background color and stroke
around the whole window.