MEGA65: implement the MIXER and use proper volume/level adjustments everywhere

[deftmega]

SID-1 seems to be 100% right instead of 66% right or similar on MEGA65. Wrong stereo placement probably applies for other SIDs aswell.

[lgblgblgb]

There is no MIXER at all implemented. However, stereo separation can be adjusted in Xemu but it's not MIXER compatible (see UI context menu, audio option). Maybe as a quick remedy, I should make the default to around 60% then? To more or less match MEGA65. Yes, still, the true MIXER implementation won't be handled by this modify, but at least a little commit which make it more close to the default MEGA65 behaviour. And I can address implementing a MIXER later, being smaller priority for now.

[lgblgblgb]

For now, I set the default stereo separation to 60% which is close enough. Later, for sure, a MEGA65 compatible MIXER should be emulated!

[lgblgblgb]

Modifying title now to reflect the bulk of the problem, have some MIXER stuff at all ....

[lgblgblgb]

Currently, implementing the mixer is lower priority because of the more ASAP stuff. However, this issue may also depends on adopting newer Hyppo, as it can be a problem to mis-match hardware features and Hyppo versions, see: #140

[lgblgblgb]

Issue #393 from @deathybrs should go here, too. The summary, also as a kind of plan:

Implement MEGA65 mixer emulation. Then, fine tune what certain "volume" and "level" settings mean for real. If not linear, with some help of a "mapping table" for example and such.

[deathybrs]

That works, I'm subscribed here now and would really like to help with this when you get around to it (primarily by analyzing the audio and numbers), provided I already have my machine.