Convert clutter_path_get_position to take a gdouble instead of an alpha value

The gdouble value represents an interval along the path from 0.0 to
1.0. This makes more sense than using an alpha value because paths are
not directly related to ClutterAlphas and the rest of the Clutter API
tends to expose gdouble arguments.

clutter/clutter-path.h

@@ -226,7 +226,7 @@ gboolean clutter_path_set_description (ClutterPath *path,
 void clutter_path_clear (ClutterPath *path);
 
 guint clutter_path_get_position (ClutterPath *path,
-                                 guint alpha,
+                                 gdouble progress,
                                  ClutterKnot *position);
 
 guint clutter_path_get_length (ClutterPath *path);