weston-log: add function to avoid direct access to compositor members in non-core code

If we use the function weston_log_context_add_log_scope()
in non-core code, it's necessary to access
weston_compositor::weston_log_ctx.

This is not ideal, since the goal is to make core structs
(weston_compositor, weston_surface, weston_output, etc)
opaque.

Add function weston_compositor_add_log_scope(), so non-core
users are able to pass weston_compositor as argument instead
of weston_compositor::weston_log_ctx.

Signed-off-by: Leandro Ribeiro <leandrohr@riseup.net>

include/libweston/weston-log.h

@@ -55,7 +55,7 @@ struct weston_debug_stream;
  *
  * @param sub The subscription.
  * @param user_data The \c user_data argument given to
- * weston_log_ctx_add_log_scope()
+ * weston_log_ctx_add_log_scope() or weston_compositor_add_log_scope().
  *
  * @memberof weston_log_scope
  */
@@ -70,6 +70,14 @@ weston_log_ctx_add_log_scope(struct weston_log_context *log_ctx,
 			     weston_log_scope_cb destroy_subscription,
 			     void *user_data);
 
+struct weston_log_scope *
+weston_compositor_add_log_scope(struct weston_compositor *compositor,
+				const char *name,
+				const char *description,
+				weston_log_scope_cb new_subscription,
+				weston_log_scope_cb destroy_subscription,
+				void *user_data);
+
 void
 weston_log_scope_destroy(struct weston_log_scope *scope);