libweston: introduce weston_output::head_list

libweston: introduce weston_output::head_list

The intention is that in the future backends will dynamically allocate
weston_heads based on the resources they have. The lifetime of a
weston_head will be independent of the lifetime of a weston_output it
may be attached to. Backends allocate objects derived from weston_head,
like they currently do for weston_output. Backend will choose when to
destroy a weston_head.

For clone mode, struct weston_output gains head_list member, which is
the list of attached heads that will all show the same framebuffer.
Since heads are growing out of weston_output, management functions are
added.

Detaching a head from an enabled output is allowed to accommodate
disappearing heads. Attaching a head to an enabled output is disallowed
because it may need hardware reconfiguration and testing, and so
requires a weston_output_enable() call.

As a temporary measure, we have one weston_head embedded in
weston_output, so that backends can be migrated individually to the new
allocation scheme.

v8:
- Do not send wp_presentation_feedback.sync_output events for multiple
  wl_output globals in weston_presentation_feedback_present().

v6:
- adapt to upstream changes in weston_output_set_transform()
- use wl_list_for_each_safe in weston_output_release()
- removed weston_output_get_first_head() as it's not needed yet

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
v5 Reviewed-by: Derek Foreman <derekf@osg.samsung.com>
v7 Reviewed-by: Ian Ray <ian.ray@ge.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Acked-by: Derek Foreman <derekf@osg.samsung.com>