Make gtk3 full_screen_toggle more robust against external changes.

The fullscreen state can also change via the window manager, so read the
state instead of trying to keep an internal flag.  All other backends
already use a similar strategy.

(`self.window` is a GtkWindow, whereas `self.window.get_window()` is a
GdkWindow...)

Author: anntzer

lib/matplotlib/backends/backend_gtk3.py

@@ -395,12 +395,10 @@ def show(self):
                 _api.warn_external("Cannot raise window yet to be setup")
 
     def full_screen_toggle(self):
-        self._full_screen_flag = not self._full_screen_flag
-        if self._full_screen_flag:
-            self.window.fullscreen()
-        else:
+        if self.window.get_window().get_state() & Gdk.WindowState.FULLSCREEN:
             self.window.unfullscreen()
-    _full_screen_flag = False
+        else:
+            self.window.fullscreen()
 
     def _get_toolbar(self):
         # must be inited after the window, drawingArea and figure