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.