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-shadow.c

@@ -77,6 +77,37 @@ st_shadow_free (StShadow *shadow)
   g_slice_free (StShadow, shadow);
 }
 
+/**
+ * st_shadow_equal:
+ * @shadow: a #StShadow
+ * @other: a different #StShadow
+ *
+ * Check if two shadow objects are identical. Note that two shadows may
+ * compare non-identically if they differ only by floating point rounding
+ * errors.
+ *
+ * Return value: %TRUE if the two shadows are identical
+ */
+gboolean
+st_shadow_equal (StShadow *shadow,
+                 StShadow *other)
+{
+  g_return_val_if_fail (shadow != NULL, FALSE);
+  g_return_val_if_fail (other != NULL, FALSE);
+
+  /* We use strict equality to compare double quantities; this means
+   * that, for example, a shadow offset of 0.25in does not necessarily
+   * compare equal to a shadow offset of 18pt in this test. Assume
+   * that a few false negatives are mostly harmless.
+   */
+
+  return (clutter_color_equal (&shadow->color, &other->color) &&
+          shadow->xoffset == other->xoffset &&
+          shadow->yoffset == other->yoffset &&
+          shadow->blur == other->blur &&
+          shadow->spread == other->spread);
+}
+
 /**
  * st_shadow_get_box:
  * @shadow: a #StShadow
@@ -105,7 +136,6 @@ st_shadow_get_box (StShadow              *shadow,
                    + shadow->blur + shadow->spread;
 }
 
-
 GType
 st_shadow_get_type (void)
 {