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.