Improve SPU envelope emulation

[sp1187]

There still appears to be some bugs in the SPU envelope emulation. Go through following in real hardware and improve emulation to match the observed behavior:

• Envelope counters, including clock timing
• Envelope target matching
• Timing of envelope
• Envelope repeat commands
• Base address register and loop control offset value
• Ramp down counters and how/if they interact with the normal envelope mode