Set Z-position correctly for wxEVT_JOY_ZMOVE events.

Z-move events don't have (x, y) position, but they do have Z-position which
needs to be set.

Also don't use (x, y) position for these events in the sample.

See #748.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70279 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/event.h

@@ -1662,11 +1662,15 @@ public:
 
     /**
         Returns the x, y position of the joystick event.
+
+        These coordinates are valid for all the events except wxEVT_JOY_ZMOVE.
     */
     wxPoint GetPosition() const;
 
     /**
         Returns the z position of the joystick event.
+
+        This method can only be used for wxEVT_JOY_ZMOVE events.
     */
     int GetZPosition() const;