Fix RP2040 I2S: always copy to output buffer

[dhalbert]

• Fixes Sinewave tone playback on RP2040 no longer working as of CP7 #5283.

Make RP2040 I2S work again.

• Don't attempt to use the sample buffer as the DMA buffer if no conversion is needed. Instead, always copy the sample buffer into the output buffer.
• Move the I2S PIO restart slightly to avoid starting before an error check.
• Add explanatory comment about getting mono->stereo conversion for free.

The quality of the I2S audio is not great: there are buzzes and noise, which happen erratically. A power cycle or reset can improve things temporarily. However, the quality seems to be the same as in CircuitPython 6.3.0. I'll open another issue about fixing the quality in the long term.

Tested with an MP3 file, a looping RawSample sine wave, and WAV files on an Adafruit Feather RP2040. Also did partial testing on a Pi Pico: no differences were found. Also smoke-tested PWMAudioOut to make sure it still works. PWM audio works better than I2S: there are fewer noisy artifacts.

Various audio testing programs I used:
audio-test-programs.zip

[kattni]

Thank you for the fix! WAV playback and tone playback are now functional on Pico. The tone playback took some tweaking but it's consistent now. WAV playback worked immediately.

Tested on Raspberry Pi Pico RP2040 with UDA1334A.