material-node: track if node owns reference on parent

MaterialNodes are used for the sparse graph of material state and layer
state. In the case of materials there is the idea of weak materials that
don't take a reference on their parent and in that case we need to be
careful not to unref our parent during
_cogl_material_node_unparent_real. This adds a has_parent_reference
member to the CoglMaterialNode struct so we now know when to skip the
unref.

clutter/cogl/cogl/cogl-material-private.h

@@ -138,6 +138,10 @@ struct _CoglMaterialNode
   /* The parent material/layer */
   CoglMaterialNode *parent;
 
+  /* TRUE if the node took a strong reference on its parent. Weak
+   * materials for instance don't take a reference on their parent. */
+  gboolean has_parent_reference;
+
   /* As an optimization for creating leaf node materials/layers (the
    * most common) we don't require any list node allocations to link
    * to a single descendant. */

clutter/cogl/cogl/cogl-material.c

@@ -127,6 +127,7 @@ _cogl_material_node_set_parent_real (CoglMaterialNode *node,
     }
 
   node->parent = parent;
+  node->has_parent_reference = take_strong_reference;
 
   /* Now that there is a consistent parent->child link we can remove
    * the parent reference if no reference was requested. If it turns
@@ -160,7 +161,8 @@ _cogl_material_node_unparent_real (CoglMaterialNode *node)
   else
     parent->children = g_list_remove (parent->children, node);
 
-  cogl_object_unref (parent);
+  if (node->has_parent_reference)
+    cogl_object_unref (parent);
 
   node->parent = NULL;
 }