Preview loaded gif frames and not wait to load all frames

Thank for making this awesome library. It's help me so much
I want Glide preview the loaded frames of gif and not waiting to load all gif frames and then preview it. Just like preview gif on Chrome browser in PC.
I use Glide version: `3.6.0`
and my code is:

```
Glide.with(this)
.load(url)
.asGif()
.diskCacheStrategy(DiskCacheStrategy.SOURCE )
.listener(new RequestListener() ....)
.into(imageview);

```

Start with the Glide request path shown in the issue, especially the .asGif() load and listener usage, and trace how GIF data becomes a GifDrawable. Determine whether frames can be displayed before the full GIF is loaded; done means an animated GIF begins previewing loaded frames without waiting for all frames.

Written by the indexing model from the issue text.