... and make it the default. This takes over the meaning from "none" for
this property in that it draws the fallback builtin image.
"none" now literally means no image will be drawn.
The code did weird things with drawing backgrounds sometimes but not
really. Now it does this:
(1) render a background
(2) render a frame
(3) render an icon
- if no icon exists, draw the generic handle icon
We want to render a background *and* the current color (if there is
one).
This also adds a custom function gtk_render_add_content_path() which
adds the path of the current content area to a cairo_t.
