py,st3m: Redraw overlays more times after mode change

Mode switching interacts weirdly with overlay drawing. Overlays
are rendered in-process, while mode change still occurs in the
graphics task. Make sure that we don't end up with framebuffer
getting cleared after rendering by queuing a few more frames
to render and flushing the whole pipeline.

python_payload/st3m/ui/elements/overlays.py

@@ -163,7 +163,7 @@ class Compositor(Responder):
             redraw = True
             self._redraw_pending -= 1
         if display_mode != self._display_mode:
-            self._redraw_pending = 2
+            self._redraw_pending = 4
         self._display_mode = display_mode
         if (display_mode & sys_display.osd) == 0:
             return