lib: add struct paddings to be able to maintain ABI compatibility

Aim for 8 pointers on each struct, wich exceptions:
- 12 pointers for already crowded/important ones
- 4 pointers for empty ones that are used for proxies; we are
  unlikely to add any virtual methods on these classes ever

docs/Doxyfile.in

@@ -2222,6 +2222,7 @@ PREDEFINED             = DOXYGEN_DOCUMENTATION_BUILD \
                          WP_API= \
                          WP_PRIVATE_API= \
                          WP_PLUGIN_EXPORT= \
+                         WP_PADDING(...)= \
                          G_GNUC_CONST= \
                          G_GNUC_PRINTF(...)= \
                          G_GNUC_NULL_TERMINATED= \

lib/wp/component-loader.h

@@ -30,6 +30,9 @@ struct _WpComponentLoaderClass
 
   gboolean (*load) (WpComponentLoader * self, const gchar * component,
       const gchar * type, GVariant * args, GError ** error);
+
+  /*< private >*/
+  WP_PADDING(6)
 };
 
 G_END_DECLS

lib/wp/defs.h

@@ -35,4 +35,6 @@
 # endif
 #endif
 
+#define WP_PADDING(n) gpointer _wp_padding[n];
+
 #endif

lib/wp/endpoint.h

@@ -28,6 +28,9 @@ G_DECLARE_DERIVABLE_TYPE (WpEndpoint, wp_endpoint, WP, ENDPOINT, WpGlobalProxy)
 struct _WpEndpointClass
 {
   WpGlobalProxyClass parent_class;
+
+  /*< private >*/
+  WP_PADDING(4)
 };
 
 WP_API

lib/wp/global-proxy.h

@@ -26,6 +26,9 @@ G_DECLARE_DERIVABLE_TYPE (WpGlobalProxy, wp_global_proxy,
 struct _WpGlobalProxyClass
 {
   WpProxyClass parent_class;
+
+  /*< private >*/
+  WP_PADDING(4)
 };
 
 WP_API

lib/wp/metadata.h

@@ -34,6 +34,9 @@ G_DECLARE_DERIVABLE_TYPE (WpMetadata, wp_metadata, WP, METADATA, WpGlobalProxy)
 struct _WpMetadataClass
 {
   WpGlobalProxyClass parent_class;
+
+  /*< private >*/
+  WP_PADDING(4)
 };
 
 WP_API

lib/wp/object.h

@@ -74,6 +74,9 @@ struct _WpObjectClass
       WpObjectFeatures missing);
 
   void (*deactivate) (WpObject * self, WpObjectFeatures features);
+
+  /*< private >*/
+  WP_PADDING(8)
 };
 
 WP_API

lib/wp/plugin.h

@@ -36,6 +36,9 @@ struct _WpPluginClass
 
   void (*enable) (WpPlugin * self, WpTransition * transition);
   void (*disable) (WpPlugin * self);
+
+  /*< private >*/
+  WP_PADDING(6)
 };
 
 WP_API

lib/wp/proxy-interfaces.h

@@ -46,6 +46,9 @@ struct _WpPipewireObjectInterface
 
   gboolean (*set_param) (WpPipewireObject * self, const gchar * id,
       guint32 flags, WpSpaPod * param);
+
+  /*< private >*/
+  WP_PADDING(5)
 };
 
 WP_API

lib/wp/proxy.h

@@ -81,6 +81,9 @@ struct _WpProxyClass
   void (*pw_proxy_destroyed) (WpProxy * self);
   void (*bound) (WpProxy * self, guint32 id);
   void (*error) (WpProxy * self, int seq, int res, const char *message);
+
+  /*< private >*/
+  WP_PADDING(6)
 };
 
 WP_API

lib/wp/session-item.h

@@ -54,6 +54,9 @@ struct _WpSessionItemClass
   void (*enable_active) (WpSessionItem * self, WpTransition * transition);
   /*! enables the exported feature of the session item */
   void (*enable_exported) (WpSessionItem * self, WpTransition * transition);
+
+  /*< private >*/
+  WP_PADDING(5)
 };
 
 /* Id */

lib/wp/si-factory.h

@@ -27,6 +27,9 @@ struct _WpSiFactoryClass
   GObjectClass parent_class;
 
   WpSessionItem * (*construct) (WpSiFactory * self, WpCore * core);
+
+  /*< private >*/
+  WP_PADDING(7)
 };
 
 WP_API

lib/wp/si-interfaces.h

@@ -32,6 +32,9 @@ struct _WpSiEndpointInterface
 
   GVariant * (*get_registration_info) (WpSiEndpoint * self);
   WpProperties * (*get_properties) (WpSiEndpoint * self);
+
+  /*< private >*/
+  WP_PADDING(6)
 };
 
 WP_API
@@ -58,6 +61,9 @@ struct _WpSiAdapterInterface
       const gchar *mode, GAsyncReadyCallback callback, gpointer data);
   gboolean (*set_ports_format_finish) (WpSiAdapter * self, GAsyncResult * res,
       GError ** error);
+
+  /*< private >*/
+  WP_PADDING(5)
 };
 
 WP_API
@@ -87,6 +93,9 @@ struct _WpSiLinkableInterface
 
   GVariant * (*get_ports) (WpSiLinkable * self, const gchar * context);
   WpSiAcquisition * (*get_acquisition) (WpSiLinkable * self);
+
+  /*< private >*/
+  WP_PADDING(6)
 };
 
 WP_API
@@ -114,6 +123,9 @@ struct _WpSiLinkInterface
 
   WpSiLinkable * (*get_out_item) (WpSiLink * self);
   WpSiLinkable * (*get_in_item) (WpSiLink * self);
+
+  /*< private >*/
+  WP_PADDING(4)
 };
 
 WP_API
@@ -148,6 +160,9 @@ struct _WpSiAcquisitionInterface
 
   void (*release) (WpSiAcquisition * self, WpSiLink * acquisitor,
       WpSiLinkable * item);
+
+  /*< private >*/
+  WP_PADDING(5)
 };
 
 WP_API

lib/wp/transition.h

@@ -43,6 +43,9 @@ struct _WpTransitionClass
   guint (*get_next_step) (WpTransition * transition, guint step);
   /*! See wp_transition_advance() */
   void (*execute_step) (WpTransition * transition, guint step);
+
+  /*< private >*/
+  WP_PADDING(6)
 };
 
 WP_API