NES: Non-linear square channel mixer option

[Yave-Yu]

Switch off non-linear would hear cleaner square channel mixing quality, but for accuracy purpose, this option is enable by default.
squareSumFactor used to correct volume for matching non-linear mixer.
Also slightly corrected non-linear mixer formula.

These game are easily test mixing quality: Atlantis no Nazo, Winter Games, Artelius, Mega Man 2, Armadillo…

[Fiskbit]

Several of our maintainers spent a bit discussing this change. The general sentiment is that we're not opposed to having meaningful enhancements in the emulator, but we are uneasy about adding really niche options that don't represent some real hardware configuration. Enhancements that address hardware limitations that get in the way of enjoying a game make a lot of sense, but as I understand it, this one seems to just be a minor change to the audio to make it sound a little different. For the average user, it's not clear why they would want to enable this option or how we should explain to them the purpose of this option.

We have plans to do a lot with audio by having selectable profiles that sound like different hardware models (see discussion in PR #37), but our focus here is definitely trying to get closer to real hardware, not further.

If we've misunderstood this change, please feel free to explain it to us in more detail. Thanks.