Synthio has fixed point overflow errors

[gamblor21]

Synthio has one (and likely more) spots where it is possible to overflow the int32_t used as an intermediary while doing fixed point math.

This function multiplies an int32_t by int16_t which can exceed the 32 bit size.

circuitpython/shared-module/synthio/__init__.c

Line 272 in ceda2f0

static void sum_with_loudness(int32_t *out_buffer32, int32_t *tmp_buffer32, int16_t loudness[2], size_t dur, int synth_chan) {

There are likely other cases as well.

The probable fix is to add a function to ensure the values are saturated after every calculation.
audiofreeverb does this

circuitpython/shared-module/audiofreeverb/Freeverb.c

Line 221 in bcf37e0

static int16_t sat16(int32_t n, int rshift) {

This issue is most likely the cause for #10200.

[gamblor21]

#10288 should have caught most of the cases. Will keep this issue open short term to see if others show up (unless others think we should close it, in case feel free).