libweston: pass backend to weston_windowed_output_api::create_head()

Pass the backend instead of the compositor to the windowed output API
create_head() method and increment the API version.

That way the backend will not have to find the backend pointer from the
compositor. This is trivial now, but in the multi-backend case would
entail iterating over all backends to find the correct one.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>