py,st3m: overlays: Initialize last_clip with the whole screen filled

This makes the rendering code clear the whole screen on first frame.
This is needed because the framebuffer may be filled with content from
previous mpy boot after a soft reboot, leading to possible artifacts.

python_payload/st3m/ui/elements/overlays.py

@@ -127,7 +127,7 @@ class Compositor(Responder):
         }
         self._last_fps_string = ""
         self._clip_rect = Region()
-        self._last_clip = Region()
+        self._last_clip = Region(-120, -120, 120, 120)
         self._display_mode = None
         self._enabled: List[Responder] = []
         self._last_enabled: List[Responder] = []