Fix: acmd dispatching discard most significant bit
Fix overflow with ramp value stepping

see bsmiles32/mupen64plus-rsp-hle@032b726#commitcomment-5241631
Refactor some mixing routines
Fix regression introduced by commit 3921889f3be47edb56b8ebc4691edbbd31d6b273

adpcm_compute_residuals should honor the count parameter.
Merge pull request #11 from bsmiles32/small_fixes

Small fixes