Add test for disposal mode 2 gifs

Tests/test_file_gif.py

@@ -1,6 +1,6 @@
 from .helper import unittest, PillowTestCase, hopper, netpbm_available
 
-from PIL import Image, ImagePalette, GifImagePlugin
+from PIL import Image, ImagePalette, GifImagePlugin, ImageDraw
 
 from io import BytesIO
 
@@ -315,6 +315,55 @@ class TestFileGif(PillowTestCase):
             img.seek(img.tell() + 1)
             self.assertEqual(img.disposal_method, i+1)
 
+    def test_dispose2_diff(self):
+        out = self.tempfile('temp.gif')
+        # 4 backgrounds: White, Grey, Black, Red
+        im_list = [
+            Image.new('RGB', (100, 100), '#fff'),
+            Image.new('RGB', (100, 100), '#999'),
+            Image.new('RGB', (100, 100), '#000'),
+            Image.new('RGB', (100, 100), '#f00'),
+        ]
+        # Red circle in center of each frame
+        for img in im_list:
+            d = ImageDraw.Draw(img)
+            d.ellipse([(40,40),(60,60)], fill='#f00')
+
+        # check per frame disposal
+        im_list[0].save(
+            out,
+            save_all=True,
+            append_images=im_list[1:],
+            disposal=2
+            )
+
+        img = Image.open(out)
+        top_left_pixels = []
+        center_pixels = []
+
+        # # Get pixel in top left
+        # rgb_img = img.convert('RGB')
+        # r, g, b = rgb_img.getpixel((1,1))
+        # top_left_pixels += [(r,g,b)]
+        # r, g, b = rgb_img.getpixel((50,50))
+        # center_pixels += [(r,g,b)]
+
+        for i in range(3):
+            rgb_img = img.convert('RGB')
+            # Get pixel in top left
+            r, g, b = rgb_img.getpixel((1,1))
+            top_left_pixels += [(r,g,b)]
+            # Get pixel in center
+            r, g, b = rgb_img.getpixel((50,50))
+            center_pixels += [(r,g,b)]
+            for prev in top_left_pixels[:i]:
+                # Change background every frame
+                self.assertNotEqual((r,g,b), prev)
+            for prev in center_pixels[:i]:
+                # Center remains red every frame
+                self.assertEqual((r,g,b), (255,0,0))
+            img.seek(img.tell() + 1)
+
     def test_iss634(self):
         img = Image.open("Tests/images/iss634.gif")
         # seek to the second frame