register( callback ) {

		if ( this.pluginCallbacks.indexOf( callback ) === - 1 ) {

			this.pluginCallbacks.push( callback );

		}

		return this;

	}
Registers a plugin callback. This API is internally used to implement the various
glTF extensions but can also used by third-party code to add additional logic
to the exporter.

@param {function(writer:GLTFWriter)} callback - The callback function to register.
@return {GLTFExporter} A reference to this exporter.