Animated GIFs don't work

[jbagot]

Hi,
I've been digging in the library and the PIL library as well and I found some things that doesn't allow to do a thumbnail of an animated GIF.

I'm trying to create a thumbnail of an animated GIF with PIL engine, debugging I saw how PIL knows that is a GIF and is using the GIFPlugin, that's good. But in the save method GIFPlugin there is this part of the code:

if not save_all or not _write_multiple_frames(im, fp, palette):
        _write_single_frame(im, fp, palette)

It's quite self-explanatory and of course is always entering inside the if and writting only a single frame, that's the main reason why is not working.
Then, there are 2 things here, first is the save_all that is impossible to be true using sorl_thumbnail library.

In the method _get_raw_data from sorl.thumbnail.engines.pil_engine.py there is this piece of code:

params = {
    'format': format_,
    'quality': quality,
    'optimize': 1,
}
...
image.save(bf, **params)

And should be here where we specify the save_all, this params should contains 'save_all': True. That quite easy...

BUT, the second part: _write_multiple_frames(im, fp, palette) it's more tricky, there is a palette var that my image doesn't have. Here is where I need some help, I'm a little bit lost in this part, someone with more experience in image managing can explain me what's this palette and what should I change to be able to run this method sucessfully?

I'll do a PR if someone help me with that, thanks.

BTW, there is another way to do a thumbnail with animated GIF? Perhaps with another engine?

Thank you!