wayland-surface: Attach the buffer to a surface, even if it doesn't have a role

Currently, set_cursor isn't properly working. Often, the requests look
like this:

  cursor_surface = wl_compositor.create_surface()
  cursor_buffer = create_cursor_buffer()
  cursor_surface.attach(cursor_buffer, 0, 0)
  cursor_surface.commit()
  wl_pointer.set_cursor(cursor_surface, 7, 14)

But since the surface doesn't "have a role" when the commit comes in,
we ignore it, and don't do anything with the pending buffer. When
set_cursor is called, however, we don't immediately update anything
since it doesn't have a buffer.

This effectively means that set_cursor has unpredictable side effects.
Weston's toy toolkit reuses the same surface for every buffer, so it
only fails the first time. In clients that use a new surface for every
cursor sprite, the cursor is effectively invisible.

To solve this, change the code to always set the buffer for a surface,
even if it doesn't have any real role.

src/wayland/meta-wayland-surface.c

@@ -250,34 +250,25 @@ ensure_buffer_texture (MetaWaylandBuffer *buffer)
 
 static void
 cursor_surface_commit (MetaWaylandSurface             *surface,
-                       MetaWaylandDoubleBufferedState *pending)
+                       MetaWaylandDoubleBufferedState *pending,
+                       gboolean                        buffer_changed)
 {
-  MetaWaylandBuffer *buffer = pending->buffer;
+  if (buffer_changed)
-
-  if (pending->newly_attached && buffer != surface->buffer_ref.buffer)
-    {
-      ensure_buffer_texture (buffer);
-      meta_wayland_buffer_reference (&surface->buffer_ref, buffer);
-    }
-
     meta_wayland_seat_update_sprite (surface->compositor->seat);
 }
 
-static gboolean
+static void
 actor_surface_commit (MetaWaylandSurface             *surface,
-                      MetaWaylandDoubleBufferedState *pending)
+                      MetaWaylandDoubleBufferedState *pending,
+                      gboolean                        buffer_changed)
 {
   MetaSurfaceActor *surface_actor = surface->surface_actor;
   MetaWaylandBuffer *buffer = pending->buffer;
-  gboolean changed = FALSE;
 
-  /* wl_surface.attach */
+  if (buffer_changed)
-  if (pending->newly_attached && buffer != surface->buffer_ref.buffer)
     {
       ensure_buffer_texture (buffer);
-      meta_wayland_buffer_reference (&surface->buffer_ref, buffer);
       meta_surface_actor_attach_wayland_buffer (surface_actor, buffer);
-      changed = TRUE;
     }
 
   surface_process_damage (surface, pending->damage);
@@ -286,15 +277,16 @@ actor_surface_commit (MetaWaylandSurface             *surface,
     meta_surface_actor_set_opaque_region (surface_actor, pending->opaque_region);
   if (pending->input_region)
     meta_surface_actor_set_input_region (surface_actor, pending->input_region);
-
-  return changed;
 }
 
 static void
 toplevel_surface_commit (MetaWaylandSurface             *surface,
-                         MetaWaylandDoubleBufferedState *pending)
+                         MetaWaylandDoubleBufferedState *pending,
+                         gboolean                        buffer_changed)
 {
-  if (actor_surface_commit (surface, pending))
+  actor_surface_commit (surface, pending, buffer_changed);
+
+  if (buffer_changed)
     {
       MetaWindow *window = surface->window;
       MetaWaylandBuffer *buffer = pending->buffer;
@@ -398,7 +390,8 @@ move_double_buffered_state (MetaWaylandDoubleBufferedState *from,
 
 static void
 subsurface_surface_commit (MetaWaylandSurface             *surface,
-                           MetaWaylandDoubleBufferedState *pending)
+                           MetaWaylandDoubleBufferedState *pending,
+                           gboolean                        buffer_changed)
 {
   /*
    * If the sub-surface is in synchronous mode, post-pone the commit of its
@@ -416,7 +409,11 @@ subsurface_surface_commit (MetaWaylandSurface             *surface,
     {
       move_double_buffered_state (pending, &surface->sub.pending_surface_state);
     }
-  else if (actor_surface_commit (surface, pending))
+  else
+    {
+      actor_surface_commit (surface, pending, buffer_changed);
+
+      if (buffer_changed)
         {
           MetaSurfaceActor *surface_actor = surface->surface_actor;
           MetaWaylandBuffer *buffer = pending->buffer;
@@ -433,6 +430,7 @@ subsurface_surface_commit (MetaWaylandSurface             *surface,
           clutter_actor_set_position (CLUTTER_ACTOR (surface_actor), x, y);
         }
     }
+}
 
 static void
 subsurface_parent_surface_committed (MetaWaylandSurface *surface);
@@ -448,13 +446,21 @@ commit_double_buffered_state (MetaWaylandSurface             *surface,
                               MetaWaylandDoubleBufferedState *pending)
 {
   MetaWaylandCompositor *compositor = surface->compositor;
+  gboolean buffer_changed = FALSE;
+
+  /* wl_surface.attach */
+  if (pending->newly_attached && pending->buffer != surface->buffer_ref.buffer)
+    {
+      meta_wayland_buffer_reference (&surface->buffer_ref, pending->buffer);
+      buffer_changed = TRUE;
+    }
 
   if (surface == compositor->seat->sprite)
-    cursor_surface_commit (surface, pending);
+    cursor_surface_commit (surface, pending, buffer_changed);
   else if (surface->window)
-    toplevel_surface_commit (surface, pending);
+    toplevel_surface_commit (surface, pending, buffer_changed);
   else if (surface->subsurface.resource)
-    subsurface_surface_commit (surface, pending);
+    subsurface_surface_commit (surface, pending, buffer_changed);
 
   g_list_foreach (surface->subsurfaces,
                   parent_surface_committed,