unstable/linux-dmabuf: add wp_linux_dmabuf_feedback

On multi-GPU setups, multiple devices can be used for rendering. Clients
need feedback about the device in use by the compositor. For instance,
if they render on another GPU, then they need to make sure the memory is
accessible between devices and that their buffers are not placed in
hidden memory.

This commit introduces a new wp_linux_dmabuf_feedback object. This
object advertises a preferred main device, a set of preferred
formats/modifiers and target devices.

Each object is bound to a wl_surface and can dynamically update its
feedback parameters. This enables fine-grained per-surface
optimizations. For instance, when a surface is scanned out on a GPU the
compositor isn't compositing with, the target device can be set to this
GPU to avoid unnecessary roundtrips.

A feedback object can also be standalone for clients that don't support
per-surface feedback.

Signed-off-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Closes: wayland#59