The OMX spec doesn't support HEVC but the OMX stack of the
zynqultrascaleplus adds it as a custom extension.
It uses the same API as the one of Android's OMX stack.
I used the H264 encoder code as a template.
https://bugzilla.gnome.org/show_bug.cgi?id=785434
The zynqultrascaleplus OMX implementation has a custom extension
allowing decoders to output dmabuf and so avoid buffers copy between OMX
and GStreamer.
Make use of this extension when built on the zynqultrascaleplus. The
buffer pool code should be re-usable for other platforms as well.
https://bugzilla.gnome.org/show_bug.cgi?id=784847
