enableExtension( name, behavior, shaderStage = this.shaderStage ) {

		const map = this.extensions[ shaderStage ] || ( this.extensions[ shaderStage ] = new Map() );

		if ( map.has( name ) === false ) {

			map.set( name, {
				name,
				behavior
			} );

		}

	}
Enables the given extension.

@param {string} name - The extension name.
@param {string} behavior - The extension behavior.
@param {string} [shaderStage=this.shaderStage] - The shader stage.