[PRQ#49079] Merge Request for ffmpeg-compat-57

MarsSeed [1] filed a request to merge ffmpeg-compat-57 [2] into ffmpeg3.4 [3]: Eliminating duplication. Also, ffmpeg-compat packages are not useful for compiling downstream dependent packages, because they lack the include headers. ffmpeg3.4 is currently maintained by me, but I'm happy to share maintainership with @HurricanePootis or others. That package declares provides=ffmpeg-compat-57, so existing binary packages can use that one just as well. [1] https://aur.archlinux.org/account/MarsSeed/ [2] https://aur.archlinux.org/pkgbase/ffmpeg-compat-57/ [3] https://aur.archlinux.org/pkgbase/ffmpeg3.4/