Re-add singlejpeg muxer

[bubbleguuum]

The singlejpeg muxer was removed in that commit, on April 10th 2021:

​https://git.ffmpeg.org/gitweb/ffmpeg.git/commit/240aa70b798e99c15191d986261966bdbce59dba

Whose text is:

"It was added in 51ac1f616f due to ticket #4218, in order to show a single
image via ffserver. With ffserver long gone, it serves no purpose."

singlejpeg is useful for generating jpeg on stdout. For example here, extracting a cover embedded in an mp3:

ffmpeg -i blargh.mp3 -map 0:1 -f singlejpeg - > /dev/null

Command above is for illustrating purpose and not super useful in itself,
but a program might want to generate a jpeg this way in memory (thus to stdout), for example to http stream it (without writing it to a file first).
I have such program that make use of singlejpeg this way.

Please consider re-adding singlejpeg.

[mkver]

Have you tried just adding -frames:v 1?

[bubbleguuum]

ffmpeg -i blargh.mp3 -frames:v 1 - > /dev/null

does not work, resulting in:

[NULL @ 0x5566b1ee6180] Unable to find a suitable output format for 'pipe:'
pipe:: Invalid argument

Writing to a file works, but this is not what I want.

Anyways, making singlejpeg available again would be great, but since I generate my own ffmpeg binaries I trivially restored it in my build.

[bubbleguuum]

This works, producing the same image than singlejpeg:

ffmpeg -i blargh.mp3 -frames:v 1 -f mjpeg - > /dev/null