Doesn't support v4l2 output

[kode54]

I can't specify -F (the unlisted output format option) to output to v4l2, ie. v4l2loopback. Even when I make the output format guessing optional, it seems it still can't initialize an encoder to use v4l2 output.

[clushie]

That is some very valuable feedback I've been trying to wrap my head around various abstraction layers and also doing disabling the guessing optional.

 h263_v4l2m2m         V4L2 mem2mem H.263 encoder wrapper (codec h263)
 h264_v4l2m2m         V4L2 mem2mem H.264 encoder wrapper (codec h264)
 hevc_v4l2m2m         V4L2 mem2mem HEVC encoder wrapper (codec hevc)
 mpeg4_v4l2m2m        V4L2 mem2mem MPEG4 encoder wrapper (codec mpeg4)
 vp8_v4l2m2m          V4L2 mem2mem VP8 encoder wrapper (codec vp8)

picking h264_v4l2m2m results in a crash for me when specifying an mp4 file