Complete meson ports for x264, ffmpeg and add to cerbero

We have meson ports for x264 and ffmpeg, and I have a branch that builds them for UWP here: https://gitlab.freedesktop.org/nirbheek/cerbero/-/commits/uwp-more-recipes-wip

Need to complete the ports so that they build correctly inside Cerbero on all platforms, enable + test optimizations, and port the recipes to use them.

Other than UWP, this will also make our Windows builds faster, and will enable debugging those plugins with Visual Studio.