Update ffmpeg version to 4.2.1
We're currently still using 4.1.4, which is missing many bugfixes and new features.
Requires gstreamer/meson-ports/ffmpeg#13 (closed) first.
We're currently still using 4.1.4, which is missing many bugfixes and new features.
Requires gstreamer/meson-ports/ffmpeg#13 (closed) first.