Add SPU2 BVOL and AVOL register write handling, and correct mixing to preserve relative volumes of channel and sound input data.

[bagelswitch]

This fixes incorrect relative volume in System 246 Vampire Night (and possibly other titles), where the BGM and dialogue was much too loud relative to SFX, by changing the following, per "SPU2_Overview_Manual" section 2.2:

• Sound data input samples are mixed with channel audio at a volume corresponding to per-core BVOL register values.
• Core0 combined output is mixed into Core1 output at a volume corresponding to Core1 AVOL register values.
• All sample mixing is changed from simple additive (+clamping), to an algorithm that preserves relative volume of both sources w/in the dynamic range.

This will resolve #1273

[jpd002]

Sorry for the long delay, but I'll be checking this in the upcoming days.

[bagelswitch]

Sorry for the long delay, but I'll be checking this in the upcoming days.

No worries! Thank you.

[jpd002]

Nice work! Thanks!