Fix handling of stereo audio within audiodelays when freq_shift=True

[relic-se]

@gamblor21 @jepler for interest.

This has been an issue that I've been aware of since the inclusion of audiodelays in 9.2.0. Echo buffer positions weren't being properly managed when handling stereo input/output. I've resolved this issue by separating the echo buffer into two halves (left + right) and offsetting the buffer where necessary. I've done my best to support single_channel_output as well, but I'm not exactly sure how to test that (I2SOut uses left/right pairs).

I've also found a minor bug when constructing audiodelays.Echo with delay_ms <= 0 || delay_ms > max_delay_ms where echo_buffer_length would not be set and remain as the default of 0. The effect of this issue wasn't too major, but I've resolved it by always setting echo_buffer_length during recalculate_delay and limiting it to the acceptable range.

Because each freq_shift mode handles the buffer differently now, I've added it so that the full echo_buffer and buffer positions are reset whenever the mode is changed to avoid potential audio abnormalities.

The example in #9874 now does not exhibit any issues. I've also tested this code with variable delay_ms values, and the frequency shifting works with full channel separation.

Fixes #9874.

[jepler]

I feel your pain about single_channel_output. As far as I can tell from source code searches this was needed by samd51 analog AudioOut but not any other audio output, and we should restructure things so that not every audio source has to bear a code size and complication cost for it.

[gamblor21]

From what I recall audio is either stored for mono in SSSS where each S is a n-bit sample or for stereo as LRLR (also n-bit samples).

I2SOut does handle taking mono samples and making them stereo by sending each sample out twice. How it does that depends on the underlying platform. For RP devices the way the PIO works it does this automatically (it is documented in the code). For SAMD I believe it does this via code.

In the code I wrote (and some of the other audio code I have seen) I just checked the channels for 1 or 2 and as long as the main buffer and echo buffer both used the same the Left/Right channels matched. But you have to make sure your total # of samples matches as well (44.1Khz would be 88.2K samples for stereo).

I have wondered if simplifying how mono/stereo and bit depth is handled would make things both easier to right and smaller, though potentially at a cost of useability on smaller / older controllers.

[gamblor21]

Code looks fine, but want to try it on the hardware. I know you wanted to review / work on #9876 first so can wait until that is done if you want.

[dhalbert]

Could you use 9.2.x as the base instead of main? We've (finally) made a 9.2.x branch. main is now for 10.0.0 Thanks.

[relic-se]

@dhalbert Thank you for the direction. I wasn't aware of that change. I'll look into making that switch soon.

[relic-se]

This PR has been replaced by #10252.