clutter/text: Plug a pipeline leak

ClutterText paints selected text using the selection_paint()
function. This function has to main branches of execution:
when the position is in the selection bound, or not. In the
former, we are leaking the CoglPipeline created by copying
the default color pipeline.

Unref the copied pipeline after using it.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3007>

clutter/clutter/clutter-text.c

@@ -1986,6 +1986,8 @@ selection_paint (ClutterText     *self,
                                        priv->cursor_rect.origin.y,
                                        priv->cursor_rect.origin.x + priv->cursor_rect.size.width,
                                        priv->cursor_rect.origin.y + priv->cursor_rect.size.height);
+
+      g_clear_pointer (&color_pipeline, cogl_object_unref);
     }
   else
     {