Panning adjustment for >1SID tunes

[ruby-R53]

It can be a bit annoying to have each SID hard-panned to the sides while still wanting stereo sound, so an option on sidplayfp.ini could be added to adjust that:

[Audio]
[...]
StereoSeparation = 50

The value would actually be in percents, where 0 means no separation (kinda useless) and 100 means full separation. On 3SID tunes, the center SID wouldn't be affected of course.

Maybe a default of 50 would be comfortable enough for headphone users (me).

[drfiemost]

Depends on libsidplayfp/libsidplayfp#41

[ruby-R53]

hmmm i was thinking of that right after i opened this issue lol

[drfiemost]

Please check libsidplayfp/libsidplayfp#167, not yet configurable but should hopefully sound better.

[ruby-R53]

damn, it really does! feels like i'm listening to one of those stereo SID remix videos; a step forward indeed :)