Image frame count

[danielebogo]

@onevcat Is it possible to store the animated image frame count when an animated image is created?

When func animatedImage(data: Data, options: ImageCreatingOptions) -> KFCrossPlatformImage? is called, the GIFAnimatedImage has the images array populated, so it's easy to just store the frame count.

If the no preloadAll or onlyFirstFrame is used, then we can retrieve the same info from the imageSource with CGImageSourceGetCount(imageSource).

SDWebImage has this value stored in the image and it's retrieved by each decoder.

I can push a PR if necessary.

[onevcat]

Thanks for it. #1647 has been merged and a new version (6.2) was released for this.