When setting up the mix matrix, don't just iterate over the first 64
(CHANNEL_BITS) positions because then we will never be able to configure
more than 64 channels in the matrix.
Instead iterate until we have filled all src and dst entries in the
matrix. For the first 64 positions we might need to check the channel
mask to get the right positions in our source matrix.
Fixes the channel mixer for >64 channels where the positions above 64
where 0 in the matrix and muted.
Fixes#5118