The spinner is a regular builtin image now. There is no need to go
through the shadows code manually anymore as regular items do get
shadows automatically.
This also allows simplifying the actual spinner drawing code so that it
actually works.
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.
