cogl/object: add cogl_clear_object utility function

This is based on `g_clear_object`, so it will be a bit more consistent
to write (and prevents the headaches from accidentally forgetting a NULL
check).

cogl/cogl/cogl-object.h

@@ -75,6 +75,20 @@ cogl_object_ref (void *object);
 void
 cogl_object_unref (void *object);
 
+/**
+ * cogl_clear_object: (skip)
+ * @object_ptr: a pointer to a #CoglObject reference
+ *
+ * Clears a reference to a #CoglObject.
+ *
+ * @object_ptr must not be %NULL.
+ *
+ * If the reference is %NULL then this function does nothing.
+ * Otherwise, the reference count of the object is decreased using
+ * cogl_object_unref() and the pointer is set to %NULL.
+ */
+#define cogl_clear_object(object_ptr) g_clear_pointer ((object_ptr), cogl_object_unref)
+
 /**
  * CoglUserDataKey:
  * @unused: ignored.