audioconvert: Fixed changing mix-matrix at runtime
Setting the property again after it had already been set ran g_value_unset() but did not initialize it again to g_value_copy() failed afterwards. Removed the unset as cleanup is done implicitely from g_value_copy().
Changing the mix-matrix property did not trigger reconfiguration of the caps, this has been added.
If the matrix is set to an empty matrix, instead of copying this the matrix is simply disabled by setting mix_matrix_is_set (formerly mix_matrix_was_set) to FALSE so the mix-matrix is ignored from now on.