surface-actor/wayland: Implement stable size and position rounding

Implement the stable rounding algorithm as described in the discussions
for the fractional-scale-v1 protocol.

This adds an override of the ClutterActor::apply_transform vfunc for
MetaSurfaceActorWayland that ensures the size and position of the
contents of the surface are rounded according to the stable rounding
algorithm.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2726>

src/wayland/meta-wayland-shell-surface.c

@@ -37,6 +37,7 @@ typedef struct _MetaWaylandShellSurfacePrivate
   MetaWindow *window;
 
   gulong unmanaging_handler_id;
+  gulong highest_scale_monitor_handler_id;
 } MetaWaylandShellSurfacePrivate;
 
 G_DEFINE_ABSTRACT_TYPE_WITH_PRIVATE (MetaWaylandShellSurface,
@@ -106,6 +107,8 @@ clear_window (MetaWaylandShellSurface *shell_surface)
 
   g_clear_signal_handler (&priv->unmanaging_handler_id,
                           priv->window);
+  g_clear_signal_handler (&priv->highest_scale_monitor_handler_id,
+                          priv->window);
   priv->window = NULL;
 
   surface_actor = meta_wayland_surface_get_actor (surface);
@@ -138,6 +141,12 @@ meta_wayland_shell_surface_set_window (MetaWaylandShellSurface *shell_surface,
 
   priv->window = window;
 
+  priv->highest_scale_monitor_handler_id =
+    g_signal_connect_swapped (window, "highest-scale-monitor-changed",
+                              G_CALLBACK (meta_wayland_surface_notify_highest_scale_monitor),
+                              surface);
+  meta_wayland_surface_notify_highest_scale_monitor (surface);
+
   surface_actor = meta_wayland_surface_get_actor (surface);
   if (surface_actor)
     clutter_actor_set_reactive (CLUTTER_ACTOR (surface_actor), TRUE);

src/wayland/meta-wayland-subsurface.c

@@ -90,6 +90,8 @@ sync_actor_subsurface_state (MetaWaylandSurface *surface)
     clutter_actor_show (actor);
   else
     clutter_actor_hide (actor);
+
+  clutter_actor_notify_transform_invalid (actor);
 }
 
 static gboolean
@@ -555,6 +557,8 @@ wl_subcompositor_get_subsurface (struct wl_client   *client,
   surface->sub.synchronous = TRUE;
   surface->protocol_state.parent = parent;
 
+  meta_wayland_surface_notify_highest_scale_monitor (surface);
+
   reference =
     g_node_last_child (parent->protocol_state.subsurface_branch_node)->data;
   queue_subsurface_placement (surface, reference,

src/wayland/meta-wayland-surface.c

@@ -2410,3 +2410,23 @@ meta_wayland_surface_is_xwayland (MetaWaylandSurface *surface)
   return FALSE;
 #endif
 }
+
+static void
+output_state_handle_highest_scale_monitor (MetaWaylandSurface *surface)
+{
+  MetaWaylandSurface *subsurface_surface;
+  MetaSurfaceActor *actor = meta_wayland_surface_get_actor (surface);
+
+  if (actor)
+    clutter_actor_notify_transform_invalid (CLUTTER_ACTOR (actor));
+
+  META_WAYLAND_SURFACE_FOREACH_SUBSURFACE (&surface->output_state,
+                                           subsurface_surface)
+    output_state_handle_highest_scale_monitor (subsurface_surface);
+}
+
+void
+meta_wayland_surface_notify_highest_scale_monitor (MetaWaylandSurface *surface)
+{
+  output_state_handle_highest_scale_monitor (surface);
+}

src/wayland/meta-wayland-surface.h

@@ -414,6 +414,8 @@ struct wl_resource * meta_wayland_surface_get_resource (MetaWaylandSurface *surf
 
 MetaWaylandCompositor * meta_wayland_surface_get_compositor (MetaWaylandSurface *surface);
 
+void meta_wayland_surface_notify_highest_scale_monitor (MetaWaylandSurface *surface);
+
 static inline MetaWaylandSurfaceState *
 meta_wayland_surface_state_new (void)
 {