enableDirective( name, shaderStage = this.shaderStage ) {

		const stage = this.directives[ shaderStage ] || ( this.directives[ shaderStage ] = new Set() );
		stage.add( name );

	}
Enables the given directive for the given shader stage.

@param {string} name - The directive name.
@param {string} [shaderStage=this.shaderStage] - The shader stage to enable the directive for.