Always auto center on 'scratchpad show' if window hasn't been repositioned by the user

This change ensures a scratchpad window is still centered on the screen
if it has first been shown on another screen of a different
resolution. Moving or resizing the scratchpad manually disables this
behavior.

include/data.h

@@ -569,6 +569,8 @@ struct Con {
     /** callbacks */
     void(*on_remove_child)(Con *);
 
+    /** not a scratchpad window, auto centered scratchpad window, or
+     * user positioned scratchpad window. */
     enum {
         SCRATCHPAD_NONE = 0,
         SCRATCHPAD_FRESH = 1,