Fix for fixed point overflow in synthio

[gamblor21]

Fix for #10286 and #10200.

There may still be more locations where fixed point could be handled better in synthio but this fix hits two big ones that I discovered so worth fixing those now.

Also refactored sat16 that will shift a fixed point result back into 16 bits out of audiofreeverb and into synthio to be used by any audio module that needs it. Potentially worth moving this to a generic "audiohelper" type module in the future in case someone builds without synthio but still wants audio effects.

[gamblor21]

I updated the tests as this change breaks most of them. I looked and as far as I could tell it is small rounding differences due to how I fixed this issue. I also listened but would appreciate it if someone else can confirm that things sound right. The rounding changes were about 1 (for ints) or less then 0.0001 for floats so not large for audio.

[relic-se]

Potentially worth moving this to a generic "audiohelper" type module in the future in case someone builds without synthio but still wants audio effects.

Not that it's necessary for this PR, but would audiocore be a reasonable location in the future?

[relic-se]

Looks clean so far. I haven't yet tested it, but I will extensively soon.

[relic-se]

Just finished running tests. No more distortion across the board!

Now that we can push Q up to much higher values, I'm a bit disappointed that it doesn't appear to want to self-oscillate like you'd expect from an analog filter. After a bit of research, I was able to find some anecdotes about biquads not being able to self-oscillate without the help of additional processing (using an envelope follower controlling gain). For our purposes, I think we can live without it.

[gamblor21]

Todbot has tested this (comment on #10200) and it fixes the issue he was having as well.

[tannewt]

Suggested change

	int16_t wi = (ring_waveform[idx] * out_buffer32[i]) / 32768; // consider for synthio_sat16 but had a weird artificat
	int16_t wi = (ring_waveform[idx] * out_buffer32[i]) / 32768; // consider for synthio_sat16 but had a weird artifact

[gamblor21]

Thanks I'll catch it my next time. It mostly is a note to remind myself to look at it more in the future anyways.

[tannewt]

One typo you may want to fix first. I don't really care so I approved.

[jepler]

thank you!