Function fire_frame_callbacks 

pub fn fire_frame_callbacks<H: ScriptHost + Resource<Mutability = Mutable>>(
    host: ResMut<'_, H>,
    due: Res<'_, DueFrame>,
    out: MessageWriter<'_, ScriptCommandEvent>,
)

Call on_frame(dt) on this host when retire_frame_request found a frame due. A host that declares no on_frame is silent, so in an app running several languages the callback reaches whichever host declared it.