FIX: Update macosx animation handling

- Emit a resize_event signal when the pixel ratio is updated. This
  enables an animation to move between hidpi and standard dpi screens
  without ruining the blitting.

- Init the draw in the animation. This is only necessary for macosx
  because an initial resize isn't emitted upon figure manager creation,
  versus the other backends where it is. This would cause the first frame
  to be stale on the macosx animations when used with blitting.

Author: greglucas

lib/matplotlib/animation.py

@@ -1199,9 +1199,16 @@ def _setup_blit(self):
         # axes
         self._blit_cache = dict()
         self._drawn_artists = []
+        # _post_draw needs to be called first to initialize the renderer
+        self._post_draw(None, self._blit)
+        # Then we need to clear the Frame for the initial draw
+        # This is typically handled in _on_resize because QT and Tk
+        # emit a resize event on launch, but the macosx backend does not,
+        # thus we force it here for everyone for consistency
+        self._init_draw()
+        # Connect to future resize events
         self._resize_id = self._fig.canvas.mpl_connect('resize_event',
                                                        self._on_resize)
-        self._post_draw(None, self._blit)
 
     def _on_resize(self, event):
         # On resize, we need to disable the resize event handling so we don't

src/_macosx.m

@@ -1329,6 +1329,8 @@ - (void)updateDevicePixelRatio:(double)scale
         goto exit;
     }
     if (PyObject_IsTrue(change)) {
+        // Notify that there was a resize_event that took place
+        gil_call_method(canvas, "resize_event");
         [self setNeedsDisplay: YES];
     }