[gtk+] renderbackground: Add some constness

[Timm Bäder <baedert src gnome org>]

commit 5e39f3c23b3c241bc51513c62dee5f71e8796a6d
Author: Timm Bäder <mail baedert org>
Date:   Tue Jan 9 06:00:57 2018 +0100

    renderbackground: Add some constness

 gtk/gtkrenderbackground.c |    8 ++++----
 1 files changed, 4 insertions(+), 4 deletions(-)
---
diff --git a/gtk/gtkrenderbackground.c b/gtk/gtkrenderbackground.c
index 612c9c0..57a0741 100644
--- a/gtk/gtkrenderbackground.c
+++ b/gtk/gtkrenderbackground.c
@@ -75,10 +75,10 @@ _gtk_theming_background_paint_color (GtkThemingBackground *bg,
 }
 
 static void
-gtk_theming_background_snapshot_color (GtkThemingBackground *bg,
-                                       GtkSnapshot          *snapshot,
-                                       const GdkRGBA        *bg_color,
-                                       GtkCssValue          *background_image)
+gtk_theming_background_snapshot_color (const GtkThemingBackground *bg,
+                                       GtkSnapshot                *snapshot,
+                                       const GdkRGBA              *bg_color,
+                                       const GtkCssValue          *background_image)
 {
   gint n_values = _gtk_css_array_value_get_n_values (background_image);
   GtkCssArea clip = _gtk_css_area_value_get