[gtk+/wip/shadows] css: Clip outset box-shadow to outside of box

[Alexander Larsson <alexl src gnome org>]

commit 8bb0ac0521e383e4a02bcdd770d9b0803e2c888a
Author: Alexander Larsson <alexl redhat com>
Date:   Fri May 3 12:50:18 2013 +0200

    css: Clip outset box-shadow to outside of box
    
    As per css3-background 7.2. Drop Shadows: the ‘box-shadow’ property:
    
      An outer box-shadow casts a shadow as if the border-box of the element
      were opaque. The shadow is drawn outside the border edge only: it
      is clipped inside the border-box of the element.
    
    Also verified vs firefox behaviour.

 gtk/gtkcssshadowvalue.c |   27 +++++++++++++++++++++------
 1 files changed, 21 insertions(+), 6 deletions(-)
---
diff --git a/gtk/gtkcssshadowvalue.c b/gtk/gtkcssshadowvalue.c
index 4e7ca65..53ab8c1 100644
--- a/gtk/gtkcssshadowvalue.c
+++ b/gtk/gtkcssshadowvalue.c
@@ -29,6 +29,8 @@
 #include "gtkthemingengineprivate.h"
 #include "gtkpango.h"
 
+#include <math.h>
+
 struct _GtkCssValue {
   GTK_CSS_VALUE_BASE
   guint inset :1;
@@ -479,30 +481,43 @@ _gtk_css_shadow_value_paint_box (const GtkCssValue   *shadow,
                                  const GtkRoundedBox *padding_box)
 {
   GtkRoundedBox box, clip_box;
-  double spread, radius;
+  double spread, radius, x, y, outside;
 
   g_return_if_fail (shadow->class == &GTK_CSS_VALUE_SHADOW);
 
   cairo_save (cr);
 
+  spread = _gtk_css_number_value_get (shadow->spread, 0);
+  radius = _gtk_css_number_value_get (shadow->radius, 0);
+  x = _gtk_css_number_value_get (shadow->hoffset, 0);
+  y = _gtk_css_number_value_get (shadow->voffset, 0);
+
   if (shadow->inset)
     {
       _gtk_rounded_box_path (padding_box, cr);
       cairo_clip (cr);
     }
+  else
+    {
+      cairo_set_fill_rule (cr, CAIRO_FILL_RULE_EVEN_ODD);
+      _gtk_rounded_box_path (padding_box, cr);
+
+      outside = spread + radius + MAX (fabs (x), fabs (y));
+      clip_box = *padding_box;
+      _gtk_rounded_box_grow (&clip_box, outside, outside, outside, outside);
+      _gtk_rounded_box_clip_path (&clip_box, cr);
+      cairo_clip (cr);
+    }
 
   box = *padding_box;
-  _gtk_rounded_box_move (&box,
-                         _gtk_css_number_value_get (shadow->hoffset, 0),
-                         _gtk_css_number_value_get (shadow->voffset, 0));
-  spread = _gtk_css_number_value_get (shadow->spread, 0);
+  _gtk_rounded_box_move (&box, x, y);
+
   if (shadow->inset)
     _gtk_rounded_box_shrink (&box, spread, spread, spread, spread);
   else /* Outset */
     _gtk_rounded_box_grow (&box, spread, spread, spread, spread);
 
   clip_box = *padding_box;
-  radius = _gtk_css_number_value_get (shadow->radius, 0);
   _gtk_rounded_box_shrink (&clip_box, -radius, -radius, -radius, -radius);
 
   cr = gtk_css_shadow_value_start_drawing (shadow, cr);