colorswatch: Render background properly

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.

gtk/gtkrenderprivate.h

@@ -22,6 +22,12 @@
 #include <pango/pango.h>
 #include <gdk/gdk.h>
 
+void        gtk_render_content_path  (GtkStyleContext   *context,
+                                      cairo_t           *cr,
+                                      double             x,
+                                      double             y,
+                                      double             width,
+                                      double             height);
 void        gtk_render_paint_spinner (cairo_t           *cr,
                                       gdouble            radius,
                                       gdouble            progress);