avmux_ivf: Add VP9 and AV1 sink capabilities
Submitted by Georg Ottinger
Link to original bug (#796718)
Description
avmux_ivf uses libav to create IVF containers.
libav's ivfenc.c supports VP9 and AV1 starting with 2018/03/12 (this is currently not a release version), but probably next release will support it.
This patch registers the capability for video/x-vp9 and video/x-av1 (along video/x-vp8) when using avmux_ivf.
This work is part of a GSOC 2018 project, mentored by Intel Media And Audio For Linux