Upstreamed Big 4th Gen Scarlett support

This release updates the FCP driver (supporting Scarlett 4th Gen 16i16, 18i16, and 18i20) to the version accepted upstream.

If you want to try this out, please contact the author by email; nearly every feature is working, but not everything is quite ready for public release yet.

If you previously tried out the v6.12f1 version then you must also update /~https://github.com/geoffreybennett/fcp-support and /~https://github.com/geoffreybennett/alsa-scarlett-gui (build from the g4-big branch).