Add a minimum support for channel layouts with a custom channels order. Usually channels are in the canonical FFmpeg order, but recently the AVChannelLayout API has been expanded to support custom orders, and it's already in use for PCM audio for example.

Needs some testing to check if everything is still working as usual.

Will fix #6300.

Tested on:

• Windows 10+ (via MinGW)
• macOS 10.13+
• Ubuntu Linux