windowManager: Replace custom shader with builtin ClutterEffects

While modal dialogs were attached to the parent's titlebar, it
made sense to leave the top of the parent window at full color.
With the new position of modal dialogs, it makes more sense to dim
the entire parent window, so we can use a combination of Clutter's
BrightnessContrast- and DesaturateEffect instead of our own custom
shader.

https://bugzilla.gnome.org/show_bug.cgi?id=674499

src/shell-util.h

@@ -39,10 +39,6 @@ gboolean shell_parse_search_provider           (const char       *data,
                                                 char            **icon_data_uri,
                                                 GError          **error);
 
-void shell_shader_effect_set_double_uniform (ClutterShaderEffect *effect,
-                                             const gchar         *name,
-                                             gdouble             value);
-
 gboolean shell_session_is_active_for_systemd (void);
 
 gboolean shell_util_wifexited                  (int               status,