similar to volume functions, simplifies leftover samples handling
for SIMD'd code path
use concrete pointer type (e.g. int16_t*) instead of void*,
saves several casts
Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
idea is to allow optimized code path (similar to volume code)
and rework/specialize mixing cases to enable runtime performance improvements
no functionality changes in this patch
Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>