Translate the source position separately

Translate the source position separately
This is to handle the case where an app specifies the same values for the
source and listener, and expects centered panning. This fails due to floating-
point errors in the matrix, causing the result to be ever-so-slightly off of 0.

This error would normally be hidden by the position normalization, which will
not lengthen a distance shorter than the reference distance so the panning
would be nearly imperceptible. But that also fails if the reference distance
is set to 0, causing the position to expand to a full unit.

Keep the 4x4 matrix calculations, however. It will still be useful for the
requested listener matrix extension.