Vaapi. The ability to add input types to the decoders plugin: GstVideoGLTextureMeta and DMABuf

[Odoacre]

> Thus, It is required to add the caps feature in vaapipostroc sink pad and its
> machinery.
>
> https://bugzilla.gnome.org/show_bug.cgi?id=797010
>
> vmjl

Ok, we will try to add it. Wouldn't it be more prudent to add a DMABuf
feature to the encoder tho? It really seems to me a bit clumsy
workaround to use an unneccessary intermediate element. I see that you
want to avoid duplication, but the post processing element before the
encoder inserted only for copyless buffer exchange seems very unobvious
and excessive.

Freyr