libweston: introduce weston_output_from_resource()

This is a simple wrapper for casting the user data of a wl_resource into
a struct weston_output pointer. Using the wrapper clearly marks all the
places where a wl_output protocol object is used.

Replace ALL wl_output related calls to wl_resource_get_user_data() with
a call to weston_output_from_resource().

v2: add type assert in weston_output_from_resource().

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Armin Krezović <krezovic.armin@gmail.com>

libweston/compositor.c

@@ -4323,6 +4323,22 @@ bind_output(struct wl_client *client,
 		wl_output_send_done(resource);
 }
 
+/** Get the backing object of wl_output
+ *
+ * \param resource A wl_output protocol object.
+ * \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)
+{
+	assert(wl_resource_instance_of(resource, &wl_output_interface,
+				       &output_interface));
+
+	return wl_resource_get_user_data(resource);
+}
+
+
 /* Move other outputs when one is resized so the space remains contiguous. */
 static void
 weston_compositor_reflow_outputs(struct weston_compositor *compositor,

libweston/compositor.h

@@ -1941,6 +1941,9 @@ 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);
+
 #ifdef  __cplusplus
 }
 #endif