create static method to transform a Jpeg into an Mpo

This is dirty "downcasting". It just works in this special case.

src/PIL/MpoImagePlugin.py

@@ -98,6 +98,22 @@ class MpoImageFile(JpegImagePlugin.JpegImageFile):
         finally:
             self.__fp = None
 
+    @staticmethod
+    def adopt(jpeg_instance, mpheader=None):
+        '''
+        Transform the instance of JpegImageFile into
+        instance of MpoImageFile.
+        After the call, the JpegImageFile is extended
+        to be an MpoImageFile.
+
+        This is essentially useful when opening a JPEG
+        file that reveals itself as an MPO, to avoid
+        double call to _open.
+        '''
+        jpeg_instance.__class__ = MpoImageFile
+        jpeg_instance._after_jpeg_open(mpheader)
+        return jpeg_instance
+
 
 # ---------------------------------------------------------------------
 # Registry stuff