Need Callbacks for when a GifDrawable with an animation finishes it's last loop

**Glide Version**: 3.8.0-SNAPSHOT

**Glide load line / `GlideModule` (if any) / list Adapter code (if any)**:
```java
glide.load(b).into(new GlideDrawableImageViewTarget(imageView, GlideDrawable.LOOP_INTRINSIC));
```

I have a series of 3 animated gifs that have to animate themselves in sequence. There seems to be no current way with glide to know when a particular animation is done so that i can start the next one.

GifDrawable seems to be the place where this might have to be

```java
@TargetApi(Build.VERSION_CODES.HONEYCOMB)
@Override
public void onFrameReady(int frameIndex) {
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB && getCallback() == null) {
stop();
reset();
return;
}

invalidateSelf();

if (frameIndex == decoder.getFrameCount() - 1) {
loopCount++;
}

if (maxLoopCount != LOOP_FOREVER && loopCount >= maxLoopCount) {
stop();
}
}
```

i could override that method, however, i'd have no access to the decoder to see if there's any more frames available after the current one.