pipeline: don't leak the layers_cache

When freeing a pipeline in _cogl_pipeline_free we weren't making sure to
free the layers_cache which was leading to a memory leak.

Thanks to Sunjin Yang for finding this.

https://bugzilla.gnome.org/show_bug.cgi?id=660986

Reviewed-by: Neil Roberts <neil@linux.intel.com>

cogl/cogl-pipeline.c

@@ -470,6 +470,8 @@ _cogl_pipeline_free (CoglPipeline *pipeline)
 
   g_list_free (pipeline->deprecated_get_layers_list);
 
+  recursively_free_layer_caches (pipeline);
+
   g_slice_free (CoglPipeline, pipeline);
 }