copy method
Implementation
@override
ShaderMaterial copy(Material source) {
super.copy(source);
fragmentShader = source.fragmentShader;
vertexShader = source.vertexShader;
uniforms = cloneUniforms(source.uniforms);
defines = json.decode(json.encode(source.defines));
wireframe = source.wireframe;
wireframeLinewidth = source.wireframeLinewidth;
fog = source.fog;
lights = source.lights;
clipping = source.clipping;
extensions = json.decode(json.encode(source.extensions));
glslVersion = source.glslVersion;
return this;
}