Merge changes I147a476c,Ia769b124,Ibc0e3346,Icc961e82,Iaeb69e03, ... into sc-dev

* changes:
  Downmix: Clean up the generic downmix
  Downmix: Remove unneeded LVM_FLOAT
  Downmix: Remove unnecessary channel mask redefine
  Downmix: Enable host test
  Downmix: Move to cpp
  Downmix: Add benchmark
  Downmix: Clean up unnecessary API and add gtest