VideoCodec.libvpx__vp9 unsupported

[MitchMakesThings]

Attempting to set the ConversionOptions VideoFormat to VideoCodec.libvpx__vp9 results in FFMpeg being called with the argument libvpx__vp9 instead of libvpx-vp9, which FFMpeg rejects as an unknown codec.

As a temporary workaround I'm leaving the VideoCodec unspecified and passing ExtraArguments = "-c:v libvpx-vp9"