constraining sample values to [-1,1] during downmixing

The downmixing equations in the spec don't preserve the property that all
sample values are in the range [-1,1]. Are the values supposed to be
normalized or clamped? Either way, it needs to be added to the spec.

Rob
-- 
Jesus called them together and said, “You know that the rulers of the
Gentiles lord it over them, and their high officials exercise authority
over them. Not so with you. Instead, whoever wants to become great among
you must be your servant, and whoever wants to be first must be your
slave — just
as the Son of Man did not come to be served, but to serve, and to give his
life as a ransom for many.” [Matthew 20:25-28]