Fix info for first frame of apng images getting clobbered when seekin…

…g to the first frame multiple times.

src/PIL/PngImagePlugin.py

@@ -378,7 +378,7 @@ def save_rewind(self):
         }
 
     def rewind(self):
-        self.im_info = self.rewind_state["info"]
+        self.im_info = self.rewind_state["info"].copy()
         self.im_tile = self.rewind_state["tile"]
         self._seq_num = self.rewind_state["seq_num"]