WiimoteEmu: Change speaker pan to use "constant power pan law".

[jordan-woyak]

This is a re-implementation of @kamiyo's PR #3474.

A constant power pan law is audibly more pleasing than linear, since linear makes signals panned to the center seem softer than those panned either all the way to the left or all the way to the right. See: http://www.rs-met.com/documents/tutorials/PanRules.pdf

-kamiyo

I've also changed the UI pan setting from a -127..127 range to -100..100.
There's no need to expose the internals of our mixer.

[kamiyo]

Thanks for taking up the mantle!

[shuffle2]

I thought dolphin was against functional-style casts? lgtm otherwise

[BhaaLseN]

They are shorter than the static_cast equivalent, but from what I read on cppreference (2) after being slightly suspicious they would be equivalent to a C-style cast? I'm pretty sure thats not what we want.

[jordan-woyak]

Yeah. But it's an integer. It's clear that it's a static cast. Static_cast is so verbose. It's sensible for pointers though.