Fix sndio raw_volume rounding

[Jzavrk]

fix #2225

Fixes + - keys in ncmpc

[MaxKellermann]

This PR contains text that is missing in the commit message; and the PR text isn't even enough because it doesn't describe what is being done and what the problem was.
Also this can be done better than reverting to floating-point math. Dividing integers with rounding is trivial enough.

[Jzavrk]

I expressed myself badly, this PR addresses mainly this issue: MusicPlayerDaemon/ncmpc#95
Floating-point math is necessary since dividing 2 integers gives no more than floor value, rendering increment impossible.
This patch has been tested before being sent to review, volume now goes effectively up and down.

[MaxKellermann]

No, floating-point math is not necessary! You don't believe me, so I have to prove it.

Look this piece of code:

return std::round(static_cast<double>(raw_volume) * 100 / SIO_MAXVOL)

1. converts integer to floating point
2. floating point multiplication by 100
3. floating point division by 127
4. invoke function round()
5. convert floating point to integer

Now compare with this one:

return (raw_volume * 100 + SIO_MAXVOL / 2) / SIO_MAXVOL

1. integer multiplication by 100
2. integer addition, adding 63
3. integer division by 127

Both functions are (almost) equal, but your version is bigger and much slower.

[Jzavrk]

Tested, it works 👍

[MaxKellermann]

Your PR now contains 2 commits, the first of which is not good. I don't want to merge known-bad commits. If you find a mistake in an unmerged commit, you should amend the commit instead of adding fixup commits.

[Jzavrk]

Forced-pushed the amended commit, I hope this one will do it.
I don't know if it has its place here but I haven't been into coding since studies (years now), first PR ever, setting up for testing on my linux desktop was not too easy (I use mpd on my FreeBSD server).
I would be sincerely curious to know what was the reasoning behind the new formulas, sounds like magic to me

[MaxKellermann]

Code is good now, but please write a proper commit message which explains the problem and how it is being solved.