Need slightly better logic for blinking gifs

It didn't take long for the spammers to start with the blinking GIF images.
Now I think they are using blinkers where the first image in the sequence is
pretty much empty.  The real content is in the second frame.  I need to
handle that.  I don't think I can just blindly overwrite one frame with the
next since they could just make the last one the blankish image.  The way I
concatenate images left-to-right and top-to-bottom makes it impossible to
just concatenate the frames together either.  Ideas?  The code is in
spambayes/ImageStripper.py in the distribution.  Look at PIL_decode_parts.

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