py,st3m-ui: hold overlays mutex for shorter intervals

For fps overlay only show every 5th frame, for the regular overlay
first collect list of active overlays, and only if there is overlays
active get and flush the overlay ctx.

Also clearing a smaller rectangle with default overlays - the fps code
which does a lot less work is more reliable for use with mpremote.

python_payload/st3m/ui/elements/overlays.py

@@ -66,6 +66,8 @@ class Compositor(Responder):
             OverlayKind.Debug: True,
             OverlayKind.Toast: True,
         }
+        self._last_fps_string = ""
+        self._frame_skip = 0
 
     def _enabled_overlays(self) -> List[Responder]:
         res: List[Responder] = []
@@ -93,8 +95,11 @@ class Compositor(Responder):
         self.main.draw(ctx)
         if (sys_display.get_mode() & sys_display.osd) == 0:
             return
-        octx = sys_display.ctx(sys_display.osd)
         if settings.onoff_show_fps.value:
+            fps_string = "{0:.1f}".format(sys_display.fps())
+            if fps_string != self._last_fps_string and self._frame_skip <= 0:
+                octx = sys_display.ctx(sys_display.osd)
+                self._last_fps_string = fps_string
                 _clip_x0 = 0
                 _clip_y1 = 0
                 _clip_x1 = 240
@@ -106,7 +111,7 @@ class Compositor(Responder):
                     _clip_x0 - 120,
                     _clip_y0 - 120,
                     _clip_x1 - _clip_x0 + 1,
-                _clip_y1 - _clip_y0 + 1,
+                    _clip_y1 - _clip_y0 + 2,
                 ).fill()
                 octx.restore()
                 octx.gray(1)
@@ -114,20 +119,35 @@ class Compositor(Responder):
                 octx.font = "Bold"
                 octx.move_to(0, -103)
                 octx.text_align = octx.CENTER
-            octx.text("{0:.1f}".format(sys_display.fps()))
+                octx.text(fps_string)
+                self._frame_skip = 5
+                sys_display.update(octx)
+                sys_display.overlay_clip(_clip_x0, _clip_y0, _clip_x1, _clip_y1)
+            else:
+                self._frame_skip -= 1
         else:
+            if self._frame_skip <= 0:
+                overlays = self._enabled_overlays()
                 _clip_x0 = 80
                 _clip_y0 = 0
                 _clip_x1 = 160
                 _clip_y1 = 0
+
+                octx = sys_display.ctx(sys_display.osd)
                 octx.save()
                 octx.compositing_mode = octx.CLEAR
+                if len(overlays) == 1:
+                    octx.rectangle(-50, -120, 100, 150).fill()
+                else:
                     octx.rectangle(-120, -120, 240, 240).fill()
                 octx.restore()
-            for overlay in self._enabled_overlays():
+                for overlay in overlays:
                     overlay.draw(octx)
-        sys_display.overlay_clip(_clip_x0, _clip_y0, _clip_x1, _clip_y1)
                 sys_display.update(octx)
+                sys_display.overlay_clip(_clip_x0, _clip_y0, _clip_x1, _clip_y1)
+                self._frame_skip = 2
+            else:
+                self._frame_skip -= 1
 
     def add_overlay(self, ov: Overlay) -> None:
         """
@@ -316,7 +336,7 @@ class OverlayVolume(Overlay):
 
         if self._showing is None:
             return
-        self._showing -= delta_ms
+        self._showing -= delta_ms / 2
         if self._showing < 0:
             self._showing = None