multimedia/ffmpeg: fork 4.* into a pinned port

Mainly for leaf consumers: applications but not plugins or libraries.
Otherwise, indirectly mixing different versions of ffmpeg libraries at
runtime can lead to crashes.

To use simply add the following before <bsd.port.mk> or <bsd.port.pre.mk>

.include "${.CURDIR:H:H}/multimedia/ffmpeg4/override.mk"

PR: 261302
Inspired by: c717faa5c764