Fix handling of stereo audio within audiodelays.Delay when freq_shift=True

[relic-se]

@gamblor21 @dhalbert for interest.

This PR has been rewritten and resubmitted from its original state (#9876) for compatibility with 9.2.x and to fix conflicts with other recent changes to audiodelays.Delay.

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.

[gamblor21]

Code looked good to me, but when I tried it and set the channels to one I lose the echo effect for freq_shift=True. It works for stereo.

[relic-se]

Code looked good to me, but when I tried it and set the channels to one I lose the echo effect for freq_shift=True. It works for stereo.

I believe there's an issue with how the buffer index is updated in mono. I fixed it within the audiodelays.MultiTapDelay effect, but I haven't tackled it here. I'll take a swing at it soon.

[relic-se]

@gamblor21 I began to encounter issues with buffer positions when using a synthio.LFO on delay_ms. In the process of working it out, I decided to implement the split buffer in stereo (1st half = left channel, 2nd half = right channel) on both delay modes (freq_shift=False/True) and always keep separate buffer positions for left and right channels. This fixes the single_channel_output problem, simplifies the code slightly, and fixes the issues I was having with the aforementioned delay_ms bug. Mono output is good from my testing as well.

[gamblor21]

Works great now. Thanks so much!