Feature: add AnimationEventView and events_iter for viewing AnimationClip events

[christopherverch]

Objective

A system needs to play animations starting at a certain time, but sometimes you don't want to skip the animation events that came before.

For example, a sword unsheathing should always play the unsheath sound, but sometimes the animation is played from a bit further into the unsheathing for a different starting pose.

Solution

This PR gives a "view" into the animation events, so that if someone needs to manually trigger them, they can. It's a bit awkward to use since it copies the internal trigger fn, but it's still useful enough that it seems worth adding.

Testing

I stuck this code into the animation_events example and it reliably manually triggers both animation events every 0.4 seconds.

fn debug_animations(
    animations: Res<Assets<AnimationClip>>,
    mut commands: Commands,
    mut timer: Local<f32>,
    time: Res<Time>,
) {
    *timer += time.delta_secs();
    if *timer > 0.4 {
        *timer = 0.0;
        for (_id, animation_clip) in animations.iter() {
            let views = animation_clip.events_iter();
            for view in views {
                dbg!(view.animation_target_id);
                dbg!(view.trigger_time);
                (view.trigger_fn)(&mut commands, Entity::PLACEHOLDER, view.trigger_time, 1.0);
            }
        }
    }
}

I understand this is a bit unwieldy, but this seems like an important thing to be able to access.

[laundmo]

This kinda implies that this is a triggerable event, could be clearer

[laundmo]

I added Needs-Design since i'm not sure this is the best solution to the problem, and i can well imagine different solutions being preferred

[christopherverch]

True on both counts, I changed the comments and I agree it is likely not the best solution, but it's definitely usable. It just seems like something that should be easily accessible and this is the best way I could find without making all the underlying structs pub.