Change tuple background to global color table index when saving

Tests/test_file_gif.py

@@ -4,6 +4,12 @@ from PIL import Image, ImagePalette, GifImagePlugin
 
 from io import BytesIO
 
+try:
+    from PIL import _webp
+    HAVE_WEBP = True
+except ImportError:
+    HAVE_WEBP = False
+
 codecs = dir(Image.core)
 
 # sample gif stream
@@ -411,6 +417,11 @@ class TestFileGif(PillowTestCase):
 
         self.assertEqual(reread.info['background'], im.info['background'])
 
+        if HAVE_WEBP and _webp.HAVE_WEBPANIM:
+            im = Image.open("Tests/images/hopper.webp")
+            self.assertIsInstance(im.info['background'], tuple)
+            im.save(out)
+
     def test_comment(self):
         im = Image.open(TEST_GIF)
         self.assertEqual(im.info['comment'],

src/PIL/GifImagePlugin.py

@@ -711,11 +711,18 @@ def _get_global_header(im, info):
         if im.info.get("version") == b"89a":
             version = b"89a"
 
+    background = 0
+    if "background" in info:
+        background = info["background"]
+        if isinstance(background, tuple):
+            # WebPImagePlugin stores an RGBA value in info["background"]
+            # So it must be converted to the same format as GifImagePlugin's
+            # info["background"] - a global color table index
+            background = im.palette.getcolor(background)
+
     palette_bytes = _get_palette_bytes(im)
     color_table_size = _get_color_table_size(palette_bytes)
 
-    background = info["background"] if "background" in info else 0
-
     return [
         b"GIF"+version +               # signature + version
         o16(im.size[0]) +              # canvas width