Added memory domains access and per-voice volume hacks in libretro

[eadmaster]

This extends #139 by adding ROM and VRAM memory domains.

It also adds some core options to rebalance the audio channels (meant for per-game user customization).

[DerKoun]

Thanks. The multiple PRs and forks happening lately are really getting me motivated to get back into things. (Though I'll have to fix the build actions first.)

If the audio channel volume settings are intended for per-game use they are perfect candidates for setting overrides: 'V' is still free and I'd use channel number times 1000 plus the percentage, e.g.:
V1050 V2100 V3000
would set channel 1 to 50%, 2 to 100% and 3 to 0%
Repeating the same letter/setting should not be a problem. So we can safe 7 letters.

What do you think @eadmaster ?

[eadmaster]

iinm, setting overrides are read from .bso files only, so it won't be possible to have the volume settings stored in a retroarch per-game option file like shown here.

E.g. some presets made for snes9x-libretro that could be easily ported.

[DerKoun]

Most setting overrides correspond to normal settings. When there is a BSO file it wins, but when there isn't the settings are set normally. So it's just an additional way of setting the settings.

I'll add the override when I upstream your work. So those who prefer it can use it.

I'll also research retroarch per-game option files. I didn't know about those before.