py,st3m: Only reset display mode when fully exiting an app

Otherwise it may get changed behind the application's back
when moving forward in the view stack.

python_payload/st3m/application.py

@@ -76,6 +76,7 @@ class Application(BaseView):
         super().on_exit()
         # set the default graphics mode, this is a no-op if
         # it is already set
+        if fully_exiting:
             sys_display.set_mode(0)
 
     def think(self, ins: InputState, delta_ms: int) -> None: