+ Refactor previous constructor to call on that new constructor

+ Reimplement is_passthrough to strictly check whether the matrix
  is an identity matrix, comparing channel-masks was incorrect:
  the mixer may be remixing from a list of positions to the same
  list of positions, but ordered differently, and reciprocally,
  the mixer may be remixing from a list of positions to another
  list of positions identically ordered

+ Remove unused tmp field, must have been a refactoring leftover

https://bugzilla.gnome.org/show_bug.cgi?id=785471