RTLPluginStatus
RTLPluginStatus:
"unavailable"|"deferred"|"requested"|"loading"|"loaded"|"error"
The possible option of the plugin's status
unavailable: Not loaded.
deferred: The plugin URL has been specified, but loading has been deferred.
requested: at least one tile needs RTL to render, but the plugin has not been set
loading: RTL is in the process of being loaded by worker.
loaded: The plugin is now loaded
error: The plugin failed to load