Upstreaming vendoring changes for Krita
Hi!
This MR is to review and possibly merge certain improvements I made to the Meson wrap of FFmpeg that GStreamer uses.
We are planning to use it as part of Krita's AppImage build process for supporting animation exports: https://invent.kde.org/graphics/krita/-/merge_requests/1599. For the record, we are aware of the performance and maintenance caveats-- however, Meson allows us to build without a hitch for most platforms.
I have tested this wrap on macOS and Windows; we're still to test Meson more broadly for Android builds.