Allow alpha differences to indicate different frames when saving GIF

Tests/test_file_gif.py

@@ -1130,6 +1130,18 @@ def test_bbox(tmp_path):
         assert reread.n_frames == 2
 
 
+def test_bbox_alpha(tmp_path):
+    out = str(tmp_path / "temp.gif")
+
+    im = Image.new("RGBA", (1, 2), (255, 0, 0, 255))
+    im.putpixel((0, 1), (255, 0, 0, 0))
+    im2 = Image.new("RGBA", (1, 2), (255, 0, 0, 0))
+    im.save(out, save_all=True, append_images=[im2])
+
+    with Image.open(out) as reread:
+        assert reread.n_frames == 2
+
+
 def test_palette_save_L(tmp_path):
     # Generate an L mode image with a separate palette
 

src/PIL/GifImagePlugin.py

@@ -569,9 +569,9 @@ def _getbbox(base_im, im_frame):
         delta = ImageChops.subtract_modulo(im_frame, base_im)
     else:
         delta = ImageChops.subtract_modulo(
-            im_frame.convert("RGB"), base_im.convert("RGB")
+            im_frame.convert("RGBA"), base_im.convert("RGBA")
         )
-    return delta.getbbox()
+    return delta.getbbox(alpha_only=False)
 
 
 def _write_multiple_frames(im, fp, palette):

src/PIL/PngImagePlugin.py

@@ -1140,7 +1140,7 @@ def _write_multiple_frames(im, fp, chunk, rawmode, default_image, append_images)
                 delta = ImageChops.subtract_modulo(
                     im_frame.convert("RGBA"), base_im.convert("RGBA")
                 )
-                bbox = delta.im.getbbox(False)
+                bbox = delta.getbbox(alpha_only=False)
                 if (
                     not bbox
                     and prev_disposal == encoderinfo.get("disposal")