Nesting AnimateOnce inside AnimationSequence causes random animation order

A user commented on this: https://forums.adafruit.com/viewtopic.php?f=60&t=186732

This (simplified) example, after the first round, does the red and blue animations in random order. Each `Pulse` animation is correct, but, successive `Pulse`s do not strictly alternate between red and blue. Note the `AnimateOnce` inside `AnimationSequence`.
```py
import board
import neopixel
from adafruit_led_animation.animation.pulse import Pulse
from adafruit_led_animation.sequence import AnimationSequence
from adafruit_led_animation.sequence import AnimateOnce
from adafruit_led_animation.color import RED, BLUE

num_pixels = 24

pixels = neopixel.NeoPixel(board.A0, num_pixels, auto_write=False)
pixels.brightness = 0.2

pulse0 = Pulse(pixels, speed=.05, color=RED, period=1)
pulse1 = Pulse(pixels, speed=.05, color=BLUE, period=1)

animations = AnimationSequence(
AnimateOnce(pulse0,pulse1,),
auto_clear=True,
auto_reset=True,
advance_interval=5,
)

while True:
animations.animate()
```
Removing the nesting works fine. Red and blue alternate, as expected.
```py
import board
import neopixel
from adafruit_led_animation.animation.pulse import Pulse
from adafruit_led_animation.sequence import AnimateOnce
from adafruit_led_animation.color import RED, BLUE

num_pixels = 24

pixels = neopixel.NeoPixel(board.A0, num_pixels, auto_write=False)
pixels.brightness = 0.2

pulse0 = Pulse(pixels, speed=.05, color=RED, period=1)
pulse1 = Pulse(pixels, speed=.05, color=BLUE, period=1)

animations = AnimateOnce(pulse0,pulse1,
auto_clear=True,
auto_reset=True,
advance_interval=5,
)

while True:
animations.animate()
```

まず adafruit_led_animation/sequence.py と AnimationSequence および AnimateOnce のエントリーポイントを確認し、次に、順序の問題を再現するために提供されたネストされた例を実行します。ネストされていない例と比較します。完了条件は、ネストされた AnimateOnce が 1 回目のラウンドの後、赤と青の Pulse アニメーションを一貫して交互に実行することです。

索引モデルが issue の本文から書いたものです。