Add options for channel selecting, flipping and mono input and output
I grouped these ideas together because they are pretty closely related:
- add an option to flip left and right channels (if there are 2 channels)
- add an option to play the left channel on both left and right output, or the right channel on both left and right output
- add an option to 'mix' stereo (2 channels) together and output the result as two identical channels (essentially turning stereo to mono without omitting either channel)
I did not mention dual-mono (2 identical input channels) because there is nothing interesting one could do with that. Dual-mono can be manipulated with the options mentioned above, it simply wont be affected by these settings in any noticeable way.
These options would fit well under a new tab in "adjustments and effects -> audio effects", together with an option to balance left and right channel output as suggested in #18546. I suggest to name this new tab "Channel Selection".