Expose current playback loop iteration

[brianguertin]

There already exists realtimeAnimationProgress, which reports the progress in the current loop iteration, but resets to 0 after each loop.

I'd also like to know what iteration we are on; how many time the animation has looped.

In the Android library, this is exposed as iteration.

It should start at 0. Every time the animation restarts due to loop mode, it will be incremented by one.

Presumably, you are already tracking this in order to support .repeat() loop mode. But I don't think it's exposed.