multimedia/ffmpeg3: New port: Legacy branch of ffmpeg 3.*

To be used exclusively with emulators/ppsspp as newer versions are
incompatible due to how the hardware in the Sony PSP works.
FFmpeg is stripped down to avoid incompatbilities with external
dependencies, only provides static libraries to avoid getting
pull in unintentionally and is statically linked to PPSSPP.

PR: 282188, 282792