[gtk+] gtkcssshadowvalue: Use the blur surface as a mask, instead of painting it

[Jasper St. Pierre <jstpierre src gnome org>]

commit 227b4a862085e233baa5de8ba143309954bae541
Author: Jasper St. Pierre <jstpierre mecheye net>
Date:   Mon Jul 28 22:04:09 2014 +0200

    gtkcssshadowvalue: Use the blur surface as a mask, instead of painting it
    
    This will make it easier to use an A8 for our blurred surface instead of
    a ARGB32, which makes things a lot easier and faster.

 gtk/gtkcssshadowvalue.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/gtk/gtkcssshadowvalue.c b/gtk/gtkcssshadowvalue.c
index 87300dc..da6d08c 100644
--- a/gtk/gtkcssshadowvalue.c
+++ b/gtk/gtkcssshadowvalue.c
@@ -366,8 +366,8 @@ gtk_css_shadow_value_finish_drawing (const GtkCssValue *shadow,
   /* Blur the surface. */
   _gtk_cairo_blur_surface (surface, radius);
 
-  cairo_set_source_surface (original_cr, surface, 0, 0);
-  cairo_paint (original_cr);
+  gdk_cairo_set_source_rgba (original_cr, _gtk_css_rgba_value_get_rgba (shadow->color));
+  cairo_mask_surface (original_cr, surface, 0, 0);
 
   cairo_destroy (cr);
   cairo_surface_destroy (surface);