ANI: Reduce Pillow frames to RGB when opaque

In some cases, such as GIF output, Pillow must convert to P (palette)
mode. Unfortunately, when done on RGBA images, this can lose some colour
information (cf, python-pillow/Pillow#6832)
But when the image starts in RGB mode, the conversion to P mode works
better, so convert frames to RGB when they don't have any transparent
pixels.

Fixes matplotlib#29190

lib/matplotlib/animation.py

@@ -492,8 +492,15 @@ def grab_frame(self, **savefig_kwargs):
         buf = BytesIO()
         self.fig.savefig(
             buf, **{**savefig_kwargs, "format": "rgba", "dpi": self.dpi})
-        self._frames.append(Image.frombuffer(
-            "RGBA", self.frame_size, buf.getbuffer(), "raw", "RGBA", 0, 1))
+        im = Image.frombuffer(
+            "RGBA", self.frame_size, buf.getbuffer(), "raw", "RGBA", 0, 1)
+        if im.getextrema()[3][0] < 255:
+            # This frame has transparency, so we'll just add it as is.
+            self._frame.append(im)
+        else:
+            # Without transparency, we switch to RGB mode, which converts to P mode a
+            # little better if needed (specifically, this helps with GIF output.)
+            self._frames.append(im.convert("RGB"))
 
     def finish(self):
         self._frames[0].save(