Notice style transitions that don't change how StThemeNode paints

Add st_theme_node_paint_equal() and use that to do two things:

 1) Avoid animating transitions where nothing changes.
 2) Copy cached painting state from the old theme node to the new
    theme node.

https://bugzilla.gnome.org/show_bug.cgi?id=627083

src/st/st-theme-node.c

@@ -2760,6 +2760,9 @@ st_theme_node_geometry_equal (StThemeNode *node,
 {
   StSide side;
 
+  g_return_val_if_fail (ST_IS_THEME_NODE (node), FALSE);
+  g_return_val_if_fail (ST_IS_THEME_NODE (other), FALSE);
+
   _st_theme_node_ensure_geometry (node);
   _st_theme_node_ensure_geometry (other);
 
@@ -2780,3 +2783,86 @@ st_theme_node_geometry_equal (StThemeNode *node,
 
   return TRUE;
 }
+
+/**
+ * st_theme_node_paint_equal:
+ * @node: a #StThemeNode
+ * @other: a different #StThemeNode
+ *
+ * Check if st_theme_node_paint() will paint identically for @node as it does
+ * for @other. Note that in some cases this function may return %TRUE even
+ * if there is no visible difference in the painting.
+ *
+ * Return value: %TRUE if the two theme nodes paint identically. %FALSE if the
+ *   two nodes potentially paint differently.
+ */
+gboolean
+st_theme_node_paint_equal (StThemeNode *node,
+                           StThemeNode *other)
+{
+  StBorderImage *border_image, *other_border_image;
+  StShadow *shadow, *other_shadow;
+  int i;
+
+  g_return_val_if_fail (ST_IS_THEME_NODE (node), FALSE);
+  g_return_val_if_fail (ST_IS_THEME_NODE (other), FALSE);
+
+  _st_theme_node_ensure_background (node);
+  _st_theme_node_ensure_background (other);
+
+  if (!clutter_color_equal (&node->background_color, &other->background_color))
+    return FALSE;
+
+  if (node->background_gradient_type != other->background_gradient_type)
+    return FALSE;
+
+  if (node->background_gradient_type != ST_GRADIENT_NONE &&
+      !clutter_color_equal (&node->background_gradient_end, &other->background_gradient_end))
+    return FALSE;
+
+  if (g_strcmp0 (node->background_image, other->background_image) != 0)
+    return FALSE;
+
+  _st_theme_node_ensure_geometry (node);
+  _st_theme_node_ensure_geometry (other);
+
+  for (i = 0; i < 4; i++)
+    {
+      if (node->border_width[i] != other->border_width[i])
+        return FALSE;
+
+      if (node->border_width[i] > 0 &&
+          !clutter_color_equal (&node->border_color[i], &other->border_color[i]))
+        return FALSE;
+
+      if (node->border_radius[i] != other->border_radius[i])
+        return FALSE;
+    }
+
+  if (node->outline_width != other->outline_width)
+    return FALSE;
+
+  if (node->outline_width > 0 &&
+      !clutter_color_equal (&node->outline_color, &other->outline_color))
+    return FALSE;
+
+  border_image = st_theme_node_get_border_image (node);
+  other_border_image = st_theme_node_get_border_image (other);
+
+  if ((border_image == NULL) != (other_border_image == NULL))
+    return FALSE;
+
+  if (border_image != NULL && !st_border_image_equal (border_image, other_border_image))
+    return FALSE;
+
+  shadow = st_theme_node_get_shadow (node);
+  other_shadow = st_theme_node_get_shadow (other);
+
+  if ((shadow == NULL) != (other_shadow == NULL))
+    return FALSE;
+
+  if (shadow != NULL && !st_shadow_equal (shadow, other_shadow))
+    return FALSE;
+
+  return TRUE;
+}