Add API to check if a GIF is animated

[nbianca]

No description provided.

[Nakilon]

What if it's animated but has one frame? Or two frames that are equal?

[nbianca]

This is a best effort approach, which tries to determine if the user would perceive it as animated (thus, if it has one frame and it is "animated", then it is no problem).

I did not try to solve the other problem because any solution I came up required full parsing of the GIF image and defeated the philosophy of the library (do things quickly with as little data as possible).

[Nakilon]

As user I would probably be ok if it was just giving the number of frames.

[sdsykes]

It would be better to call the api "animated?" and return a boolean value.

if FastImage.animated?(gif_file)
  ...do something
end

[sdsykes]

Hi, looking good.

Fix the inline documentation to match?

[nbianca]

Hi @sdsykes! I've made the necessary changes to match documentation. Can you have another look? 😊

[sdsykes]

Thanks I think this is ok, I will final check and merge/release soon.