Channel numbers

These constants are meant to be used with Chs to indicate a specific number of channels to use.

In CHANNELS_ constants, the first number is the number of standard audio channels, and the second number is the number of subwoofers.

In CHANNELS_ATMOS_ constants, intended for use in Dolby Atmos systems, the first number is the number of standard audio channels, the second number is the number of subwoofers, and the third number is the number of overhead speakers.