wayland: Clear tablet tool cursor on proximity out

This ensures that the tablet tool will get the cursor updated even if it
happens to fall within the same window again on the next proximity in.

https://bugzilla.gnome.org/show_bug.cgi?id=785375

gdk/wayland/gdkdevice-wayland.c

@@ -3631,6 +3631,7 @@ tablet_tool_handle_proximity_out (void                      *data,
   tablet->pointer_info.focus = NULL;
 
   gdk_device_update_tool (tablet->current_device, NULL);
+  g_clear_object (&tablet->pointer_info.cursor);
 }
 
 static void