Implement audiomixer.MixerVoice.level as synthio.BlockInput

[relic-se]

Allow tremolo-style effects using audiomixer.MixerVoice by implementing block input on level property.

LFO ticking is not completely implemented yet because it relies on changes to shared_bindings_synthio_lfo_tick introduced in #9776 to allow for a duration less than SYNTHIO_MAX_DUR.

Demonstration:

import audiobusio
import audiocore
import audiomixer
import board
import synthio
import time

DELAY = 3.0

wave_file = open("StreetChicken.wav", "rb")
wave = audiocore.WaveFile(wave_file)

mixer = audiomixer.Mixer(
    voice_count=1,
    sample_rate=wave.sample_rate,
    channel_count=wave.channel_count,
)

audio = audiobusio.I2SOut(
    bit_clock=board.GP3,
    word_select=board.GP4,
    data=board.GP5,
)

audio.play(mixer)
mixer.play(wave, loop=True)

print("full")
time.sleep(DELAY)
mixer.voice[0].level = 0.5
print("half")
time.sleep(DELAY)
mixer.voice[0].level = synthio.LFO(rate=0.5, scale=0.5, offset=0.5)
print("lfo")
time.sleep(DELAY)
mixer.voice[0].stop()