Convert the channels present in channel_mask to a position array
(which should have at least channels entries ensured by caller).
If channel_mask is set to 0, it is considered as 'not present' for purpose
of conversion.
A partially valid channel_mask with less bits set than the number
of channels is considered valid.
Convert the
channelspresent inchannel_maskto apositionarray (which should have at leastchannelsentries ensured by caller). Ifchannel_maskis set to 0, it is considered as 'not present' for purpose of conversion. A partially validchannel_maskwith less bits set than the number of channels is considered valid.