In the future we want to have the ffmpeg backend default on all platforms, se we need to start integrating ffmpeg on iOS.

The expected result of the task is compilation of ffmpeg and QtMM+ffmpeg on CI, the darwin backend should still be the default (until we test ffmpeg carefully).

Some details.

Ffmpeg compilation/installation scripts:

coin/provisioning/%platform%/90-install-ffmpeg.sh
coin/provisioning/common/unix/install-ffmpeg.sh

Common ffmpeg build config (might be modified for iOS)
coin/provisioning/common/shared/ffmpeg_config_options.txt

Selection of the actual media backend
runtime: env var QT_MEDIA_BACKEND=darwin, QT_MEDIA_BACKEND=ffmpeg
compile-time: cmake var QT_DEFAULT_MEDIA_BACKEND=...