Add a warning when ClutterInterval can't compute progress

This is really useful when trying to animate GTypes that haven't
registered any progress function. Instead of silently not working it
will warn the developer.

http://bugzilla.openedhand.com/show_bug.cgi?id=1845

clutter/clutter-interval.c

@@ -307,6 +307,15 @@ clutter_interval_real_compute_value (ClutterInterval *interval,
       break;
     }
 
+  /* We're trying to animate a property without knowing how to do that. Issue
+   * a warning with a hint to what could be done to fix that */
+  if (G_UNLIKELY (retval == FALSE))
+      g_warning ("%s: Could not compute progress between two %ss. You can "
+                 "register a progress function to instruct ClutterInterval "
+                 "how to deal with this GType",
+                 G_STRLOC,
+                 g_type_name (value_type));
+
   return retval;
 }