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
https://gitlab.freedesktop.org/wayland/wayland/issues/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>

protocol/meson.build

@@ -1,7 +1,10 @@
 dep_scanner = dependency('wayland-scanner', native: true)
 prog_scanner = find_program(dep_scanner.get_pkgconfig_variable('wayland_scanner'))
 
-dep_wp = dependency('wayland-protocols', version: '>= 1.14')
+# XXX: For minor version 2 of zwp_linux_explicit_synchronization_v1, we
+# actually need a development version after 1.17, but there is no way to
+# express such a requirement at the moment.
+dep_wp = dependency('wayland-protocols', version: '>= 1.17')
 dir_wp_base = dep_wp.get_pkgconfig_variable('pkgdatadir')
 
 install_data(
@@ -18,6 +21,7 @@ generated_protocols = [
 	[ 'ivi-hmi-controller', 'internal' ],
 	[ 'fullscreen-shell', 'v1' ],
 	[ 'linux-dmabuf', 'v1' ],
+	[ 'linux-explicit-synchronization', 'v1' ],
 	[ 'presentation-time', 'stable' ],
 	[ 'pointer-constraints', 'v1' ],
 	[ 'relative-pointer', 'v1' ],