Adds the `whip_output_audio` and `whip_output_video` output kinds,
which selectively advertise only video or only audio support.
To use these types, it is effectively the same as with the AV
version. Just create the output, assign your video or audio encoder,
and you're good.
libobs does not have support for "optional" outputs. With an AV output,
if you only assign a video or audio encoder, start will fail.
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