Do not rearrange palette channels

Author: radarhere

Tests/test_file_gif.py

@@ -833,7 +833,7 @@ def test_palette_save_ImagePalette(tmp_path):
 
     with Image.open(out) as reloaded:
         im.putpalette(palette)
-        assert_image_equal(reloaded, im)
+        assert_image_equal(reloaded.convert("RGB"), im.convert("RGB"))
 
 
 def test_save_I(tmp_path):

src/PIL/GifImagePlugin.py

@@ -396,15 +396,7 @@ def _normalize_palette(im, palette, info):
         if isinstance(palette, (bytes, bytearray, list)):
             source_palette = bytearray(palette[:768])
         if isinstance(palette, ImagePalette.ImagePalette):
-            source_palette = bytearray(
-                itertools.chain.from_iterable(
-                    zip(
-                        palette.palette[:256],
-                        palette.palette[256:512],
-                        palette.palette[512:768],
-                    )
-                )
-            )
+            source_palette = bytearray(palette.palette)
 
     if im.mode == "P":
         if not source_palette: