Vaapi. The ability to add input types to the decoders

[Odoacre]

> Sorry, atm it requires the caps feature and we have issues there since the
> dmabuf are tiled, event though the modifiers aren't set by mesa. The mesa
> dmabuf exporter is massively broken, so we'll have to disable this really
> soon in libgstgl. It work till around Mesa 1.17, when the tiling and
> compression got activated by default for all internal texture backend.

Does it mean that the only reliable way of passing a GPU memory object
between the vaapi and GL plugins is GstVideoGLTextureUploadMeta? Does it
imply a copyless GPU memory object passing? If so, we could implement it
for the vaapi encoder plugins, since it's already implemented in vaapi
decoders anyway. AFAIU vaapi people (as well as gstreamer community in
general) are leaning towards the DMABuf-based solutions, but since it
does not work maybe we should implement GstVideoGLTextureUploadMeta
approach? It's really a shame that we have to do so much unneccessary
copying between GL and vaapi elements.

Freyr