libweston: make wl_output point to weston_head

The user data of a wl_resource representing a wl_output protocol object
used to be a pointer to weston_output. Now that weston_output is being
split, wl_output more accurately refers to weston_head which is a single
monitor.

Change the wl_output user data to point to weston_head.
weston_output_from_resource() is replaced with
weston_head_from_resource().

This change is not strictly necessary, but architecturally it is the
right thing to do. In the future there might appear the need to refer to
a specific head of a cloned pair, for instance.

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

libweston/compositor.c

@@ -4330,7 +4330,7 @@ bind_output(struct wl_client *client,
 	}
 
 	wl_list_insert(&head->resource_list, wl_resource_get_link(resource));
-	wl_resource_set_implementation(resource, &output_interface, output,
+	wl_resource_set_implementation(resource, &output_interface, head,
 				       unbind_resource);
 
 	assert(output);
@@ -4364,8 +4364,8 @@ bind_output(struct wl_client *client,
  * \return The backing object (user data) of a wl_resource representing a
  * wl_output protocol object.
  */
-WL_EXPORT struct weston_output *
-weston_output_from_resource(struct wl_resource *resource)
+WL_EXPORT struct weston_head *
+weston_head_from_resource(struct wl_resource *resource)
 {
 	assert(wl_resource_instance_of(resource, &wl_output_interface,
 				       &output_interface));

libweston/compositor.h

@@ -1995,8 +1995,8 @@ weston_output_disable(struct weston_output *output);
 void
 weston_pending_output_coldplug(struct weston_compositor *compositor);
 
-struct weston_output *
-weston_output_from_resource(struct wl_resource *resource);
+struct weston_head *
+weston_head_from_resource(struct wl_resource *resource);
 
 #ifdef  __cplusplus
 }