libweston: Introduce zwp_linux_explicit_synchronization_v1

Introduce support for the zwp_linux_explicit_synchronization_unstable_v1
protocol with an implementation of the zwp_linux_explicit_synchronization_v1
interface.

Explicit synchronization provides a more versatile notification
mechanism for buffer readiness and availability, and can be used to
improve efficiency by integrating with related functionality in display
and graphics APIs.

In addition, the per-commit nature of the release events provided by
this protocol potentially offers a solution to a deficiency of the
wl_buffer.release event (see
wayland/wayland#46

).

Support for this protocol depends on the capabilities of the backend, so
we don't register it by default but provide a function which each
backend will need to call. In this commit only the headless backend when
using the noop renderer supports this to enable testing.

Note that the zwp_surface_synchronization_v1 interface, which contains
the core functionality of the protocol, is not implemented in this
commit. Support for it will be added in future commits.

Changes in v7:
  - Added some information in the commit message about the benefits of
    the explicit sync protocol.

Changes in v6:
  - Fall back to advertising minor version 1 of the explicit sync protocol,
    although we support minor version 2 features, until the new
    wayland-protocols version is released.

Changes in v5:
  - Meson support.
  - Advertise minor version 2 of the explicit sync protocol.

Changes in v4:
  - Enable explicit sync support in the headless backend for all
    renderers.

Changes in v3:
  - Use wl_resource_get_version() instead of hardcoding version 1.
  - Use updated protocol interface names.
  - Use correct format specifier for resource id.
  - Change test name to 'linux-explicit-synchronization.weston'
    (s/_/-/g).

Changes in v2:
  - Move implementation to separate file so protocol can be registered
    on demand by backends.
  - Register protocol in headless+noop backend for testing purposes.

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>