copy( source ) {

		super.copy( source );

		this.fragmentShader = source.fragmentShader;
		this.vertexShader = source.vertexShader;

		this.uniforms = cloneUniforms( source.uniforms );
		this.uniformsGroups = cloneUniformsGroups( source.uniformsGroups );

		this.defines = Object.assign( {}, source.defines );

		this.wireframe = source.wireframe;
		this.wireframeLinewidth = source.wireframeLinewidth;

		this.fog = source.fog;
		this.lights = source.lights;
		this.clipping = source.clipping;

		this.extensions = Object.assign( {}, source.extensions );

		this.glslVersion = source.glslVersion;

		return this;

	}
Defines the GLSL version of custom shader code.

@type {?(GLSL1|GLSL3)}
@default null