settings: Provide a default style cascade per scale

This guarantees we only create 1 extra style cascade in total for hidpi
and not one per style context.

Style cascades are now nested like this:

GtkSettings root cascade (scale == 1)
 |
 +-- GtkSettings per scale cascade (for any scale, no custom providers)
 |
 +-- GtkStyleContext custom cascade (for any scale, custom providers)

This requires a bunch of care when changing cascade-related properties
inside GtkStyleContext, so that it ends up with a properly setup
cascade, but I think I got those cases right.

The only thing we don't do yet is reverting to a GtkSettings cascade
when the last custom provider is removed from a custom cascade.

gtk/gtksettingsprivate.h

@@ -38,7 +38,8 @@ gboolean            _gtk_settings_parse_convert              (GtkRcPropertyParse
                                                               GParamSpec             *pspec,
                                                               GValue                 *dest_value);
 GdkScreen          *_gtk_settings_get_screen                 (GtkSettings            *settings);
-GtkStyleCascade    *_gtk_settings_get_style_cascade          (GtkSettings            *settings);
+GtkStyleCascade    *_gtk_settings_get_style_cascade          (GtkSettings            *settings,
+                                                              gint                    scale);
 
 typedef enum
 {