Fix TripleOscillator's white noise generator

[LostRobotMusic]

TripleOscillator's noise generator has a major bug which causes it to sound atrociously ugly when multiple notes are played simultaneously, due to the random number generator it uses. This PR switches it from fast_rand to rand to fix this.

To reproduce the bug, open TripleOscillator, set any oscillator to white noise, and play any chord.

(This PR technically breaks backwards compatibility, but that really shouldn't matter here.)

[DomClark]

Note that there's a CodeFactor warning introduced here:

Consider using rand_r(...) instead of rand(...) for improved thread safety. (runtime/threadsafe_fn)

Since fast_rand isn't thread-safe either, I wouldn't consider this a regression. However, we ought to consider a better approach to pseudo-random number generation in the future (for the whole program in general, not just this function here). The generators in the C++ standard library would be worth considering (not least because rand_r is a POSIX function, rather than standard C/C++).

[sakertooth]

However, we ought to consider a better approach to pseudo-random number generation in the future (for the whole program in general, not just this function here). The generators in the C++ standard library would be worth considering (not least because rand_r is a POSIX function, rather than standard C/C++).

I agree. I made similar suggestions to this #6466, but this is fine for now.