let user run a function on a scene entities

[mockersf]

make InstanceId of a spawned scene available
check that a scene instance has been spawned (it takes 1 frame for the scene to spawn + x frames while the asset (gltf) is loaded)
run a function on entities from a spawned scene

also added an example for this PR and #1026

[cart]

Thoughts on removing for_entity_in_scene_instance in favor of:

fn iter_instance_entities(&self, instance_id: InstanceId) -> 
  Option<impl Iterator<Item=Entity>>  {
  self.spawned_instances
    .get(&instance_id)
    .map(|instance| instance.entity_map.values.cloned())
}

[cart]

I think it would make it a bit more flexible

[mockersf]

good idea! I tried using the same idea as for_current_entity but this seems better

[cart]

Yeah i'm not a huge fan of for_current_entity, but the builder pattern sort of forced my hand / i consider it to be an exception rather than a rule. SceneSpawner isn't a builder so I don't feel constrained in the same way.

[mockersf]

changed to provide iter_instance_entities, and also a funnier example to look at

[CleanCut]

@mockersf On my Intel Mac I see only a blank gray window without any console errors when I run cargo run --example update_gltf_scene -- same thing if I add --release or try it on the current master.

[mockersf]

@CleanCut oh drat, it's because of a PR mix between this one and #1020, fixed in #1204...