multimedia/ffmpeg: convert SDL option to subpackage

[jbeich]

Summary

Similar to D43612. Requested in bug 253545.

Test Plan

$ poudriere testport -j 132amd64 multimedia/ffmpeg~sdl
[...]
===========================================================================
=>> Checking shared library dependencies
 0x0000000000000001 NEEDED               Shared library: [libSDL2-2.0.so.0]
 0x0000000000000001 NEEDED               Shared library: [libavcodec.so.60]
 0x0000000000000001 NEEDED               Shared library: [libavdevice.so.60]
 0x0000000000000001 NEEDED               Shared library: [libavfilter.so.9]
 0x0000000000000001 NEEDED               Shared library: [libavformat.so.60]
 0x0000000000000001 NEEDED               Shared library: [libavutil.so.58]
 0x0000000000000001 NEEDED               Shared library: [libc.so.7]
 0x0000000000000001 NEEDED               Shared library: [libm.so.5]
 0x0000000000000001 NEEDED               Shared library: [libpostproc.so.57]
 0x0000000000000001 NEEDED               Shared library: [libswresample.so.4]
 0x0000000000000001 NEEDED               Shared library: [libswscale.so.7]
 0x0000000000000001 NEEDED               Shared library: [libthr.so.3]

$ poudriere testport -j 132amd64 multimedia/ffmpeg
[...]
===========================================================================
=>> Checking shared library dependencies
 0x0000000000000001 NEEDED               Shared library: [libSDL2-2.0.so.0]
 0x0000000000000001 NEEDED               Shared library: [libSvtAv1Enc.so.1]
 0x0000000000000001 NEEDED               Shared library: [libX11.so.6]
 0x0000000000000001 NEEDED               Shared library: [libaom.so.3]
 0x0000000000000001 NEEDED               Shared library: [libass.so.9]
 0x0000000000000001 NEEDED               Shared library: [libavcodec.so.60]
 0x0000000000000001 NEEDED               Shared library: [libavdevice.so.60]
 0x0000000000000001 NEEDED               Shared library: [libavfilter.so.9]
 0x0000000000000001 NEEDED               Shared library: [libavformat.so.60]
 0x0000000000000001 NEEDED               Shared library: [libavutil.so.58]
 0x0000000000000001 NEEDED               Shared library: [libbz2.so.4]
 0x0000000000000001 NEEDED               Shared library: [libc.so.7]
 0x0000000000000001 NEEDED               Shared library: [libdav1d.so.7]
 0x0000000000000001 NEEDED               Shared library: [libdrm.so.2]
 0x0000000000000001 NEEDED               Shared library: [libfontconfig.so.1]
 0x0000000000000001 NEEDED               Shared library: [libfreetype.so.6]
 0x0000000000000001 NEEDED               Shared library: [libgmp.so.10]
 0x0000000000000001 NEEDED               Shared library: [libgnutls.so.30]
 0x0000000000000001 NEEDED               Shared library: [libharfbuzz.so.0]
 0x0000000000000001 NEEDED               Shared library: [libjxl.so.0.9]
 0x0000000000000001 NEEDED               Shared library: [libjxl_threads.so.0.9]
 0x0000000000000001 NEEDED               Shared library: [liblcms2.so.2]
 0x0000000000000001 NEEDED               Shared library: [liblzma.so.5]
 0x0000000000000001 NEEDED               Shared library: [libm.so.5]
 0x0000000000000001 NEEDED               Shared library: [libmp3lame.so.0]
 0x0000000000000001 NEEDED               Shared library: [libopus.so.0]
 0x0000000000000001 NEEDED               Shared library: [libplacebo.so.338]
 0x0000000000000001 NEEDED               Shared library: [libpostproc.so.57]
 0x0000000000000001 NEEDED               Shared library: [libshaderc_shared.so.1]
 0x0000000000000001 NEEDED               Shared library: [libswresample.so.4]
 0x0000000000000001 NEEDED               Shared library: [libswscale.so.7]
 0x0000000000000001 NEEDED               Shared library: [libthr.so.3]
 0x0000000000000001 NEEDED               Shared library: [libv4l2.so.0]
 0x0000000000000001 NEEDED               Shared library: [libva-drm.so.2]
 0x0000000000000001 NEEDED               Shared library: [libva-x11.so.2]
 0x0000000000000001 NEEDED               Shared library: [libva.so.2]
 0x0000000000000001 NEEDED               Shared library: [libvdpau.so.1]
 0x0000000000000001 NEEDED               Shared library: [libvmaf.so.3]
 0x0000000000000001 NEEDED               Shared library: [libvorbis.so.0]
 0x0000000000000001 NEEDED               Shared library: [libvorbisenc.so.2]
 0x0000000000000001 NEEDED               Shared library: [libvpx.so.9]
 0x0000000000000001 NEEDED               Shared library: [libwebp.so.7]
 0x0000000000000001 NEEDED               Shared library: [libwebpmux.so.3]
 0x0000000000000001 NEEDED               Shared library: [libx264.so.164]
 0x0000000000000001 NEEDED               Shared library: [libx265.so.200]
 0x0000000000000001 NEEDED               Shared library: [libxcb-shape.so.0]
 0x0000000000000001 NEEDED               Shared library: [libxcb-shm.so.0]
 0x0000000000000001 NEEDED               Shared library: [libxcb-xfixes.so.0]
 0x0000000000000001 NEEDED               Shared library: [libxcb.so.1]
 0x0000000000000001 NEEDED               Shared library: [libxml2.so.2]
 0x0000000000000001 NEEDED               Shared library: [libz.so.6]

[jbeich]

Unsuitable for subpackaging:

$ ffmpeg
ld-elf.so.1: Shared object "libSDL2-2.0.so.0" not found, required by "libavdevice.so.60"

$ ldd -a `which ffmpeg`
[...]
/usr/local/lib/libavdevice.so.60:
[...]
        libSDL2-2.0.so.0 => not found (0)
[...]