cssshadow: plug a cairo_surface_t leak

We were never destroying the cairo surface we use for blurring, which
would lead to a huge leak.

https://bugzilla.gnome.org/show_bug.cgi?id=686209

gtk/gtkcssshadowvalue.c

@@ -363,6 +363,7 @@ gtk_css_shadow_value_finish_drawing (const GtkCssValue *shadow,
   cairo_paint (original_cr);
 
   cairo_destroy (cr);
+  cairo_surface_destroy (surface);
 
   return original_cr;
 }