Implement a basic version of linux-dmabuf-unstable-v1 version 4.
Only default hints are implemented.
The new wlr_linux_dmabuf_feedback_v1 data structure will allow
compositors to define their own custom hints in the future. This
data structure makes it easy to describe feedback metadata.
It's converted to a "compiled" form suitable for marshalling over
the Wayland socket via feedback_compile.