lavr: print out the mix matrix in ff_audio_mix_set_matrix()

This will print the new matrix if it is set after initialization.
lavr: make sure that the mix function is reset even if no mixing will be done

If the matrix reduction ends up with no mixing matrix needed, we need to still
reset the mix function accordingly and log the info to the user.
lavr: cosmetics: reindent
lavr: fix matrix reduction for upmixing in certain cases

Do not skip an output if the corresponding input contributes to other output
channels.
lavr: fix mixing matrix reduction when normalization is disabled

In some cases when an input contributes fully to the corresponding
output, other inputs may also contribute to the same output. This is the
case, for example, for the default 5.1 to stereo downmix matrix without
normalization.