Added mono to stereo conversion and panning to audiomixer.Mixer

[relic-se]

Allow mono audiosample sources to play through a stereo audiomixer.Mixer object and add audiomixer.MixerVoice.panning (with synthio.BlockInput support).

These new features greatly improve audiomixer.Mixer usability for sample-based musical instruments. It also optimizes flash storage when stereo audio files are unnecessary when mixed with a stereo source (you'd otherwise have to double the channel count + file size).

Code Demonstration (requires appropriate mono.wav and stereo.wav files):

import board
import audiobusio
import audiocore
import audiomixer
import synthio

wav_stereo = audiocore.WaveFile("/stereo.wav")
wav_mono = audiocore.WaveFile("/mono.wav")
if wav_stereo.channel_count != 2 or wav_mono.channel_count != 1:
    raise ValueError("audio file channel counts are incorrect")

audio = audiobusio.I2SOut(
    bit_clock=board.GP20,
    word_select=board.GP21,
    data=board.GP22,
)

mixer = audiomixer.Mixer(
    voice_count=2,
    channel_count=2,
    sample_rate=wav_stereo.sample_rate,
    bits_per_sample=wav_stereo.bits_per_sample,
)

for i in range(len(mixer.voice)):
    mixer.voice[i].level = 0.5
    mixer.voice[i].panning = synthio.LFO(rate=0.5*(i+1))

audio.play(mixer)

mixer.voice[0].play(wav_stereo, loop=True)
mixer.voice[1].play(wav_mono, loop=True)

Closes #4463

[relic-se]

It looks like a few atmel-samd boards are running out of flash firmware space. Would it be better for me to disable this feature (panning and mono-to-stereo) on those boards or disable a larger existing feature?

The following boards are affected:

• feather_m4_express
• itsybitsy_m4_express
• metro_m4_express
• seeeduino_wio_terminal

[gamblor21]

The code looks fine to me. I have not yet tested it on hardware. Not sure offhand how to free up a few bytes, it isn't overflowing by much. I'll leave that to others who may know more about it.

[relic-se]

@dhalbert Apologies if my branch sync messed something up, but it looks like all checks are clear now. Thank you for your work on those boards. Let me know if there's anything else I can help with to get this over the finish line. 👍

[dhalbert]

@relic-se Nothing wrong! I just shrunk some boards and will now merge.

[dhalbert]

Thank you!