have individual function for mixing stream with different sample format instead
of huge case block in pa_mix()
shorter functions, prepare for optimized code path
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>