libweston: Let multiple backends register the Windowed Output API

The windowed output API is implemented by the Wayland, the X11 and the
headless backends. It's currently not possible to create a secondary
headless backend when the primary backend is Wayland or X11 because
the windowed output API would be registered twice. This commit
suffixes the windowed output API names with the backend name in order
to avoid clashes: "weston_windowed_output_api_<backend>_v2".

A use case for Wayland or X11 as primary backend and headless as
secondary is for instance to request output captures on the headless
backend to avoid read backs on the primary backend's render buffers.

Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>