GIF: Clear the buffer to the correct initial color before decoding frame 0.

This ensures that transparent pixels in the first frame data clear to
transparency and not to some previously decoded frame.

Author: zewt

src/PIL/GifImagePlugin.py

@@ -286,6 +286,16 @@ def _seek(self, frame):
         if self.palette:
             self.mode = "P"
 
+    def load_prepare(self):
+        super(GifImageFile, self).load_prepare()
+
+        if self.__frame == 0:
+            # On the first frame, clear the buffer.  Use the transparency index
+            # if we have one, otherwise use the background index.
+            default_color = self.info.get("transparency",
+                            self.info.get("background", 0))
+            self.im.paste(default_color, (0,0, self.size[0], self.size[1]))
+
     def tell(self):
         return self.__frame