lib: return the same object in libvlc_retain()

include/vlc/libvlc.h

@@ -181,8 +181,9 @@ LIBVLC_API void libvlc_release( libvlc_instance_t *p_instance );
  * The initial reference count is 1 after libvlc_new() returns.
  *
  * \param p_instance the instance to reference
+ * \return the same object
  */
-LIBVLC_API void libvlc_retain( libvlc_instance_t *p_instance );
+LIBVLC_API libvlc_instance_t *libvlc_retain( libvlc_instance_t *p_instance );
 
 /**
  * Get the ABI version of the libvlc library.

lib/core.c

@@ -82,11 +82,12 @@ error:
     return NULL;
 }
 
-void libvlc_retain( libvlc_instance_t *p_instance )
+libvlc_instance_t *libvlc_retain( libvlc_instance_t *p_instance )
 {
     assert( p_instance != NULL );
 
     vlc_atomic_rc_inc( &p_instance->ref_count );
+    return p_instance;
 }
 
 void libvlc_release( libvlc_instance_t *p_instance )