copy method
Implementation
@override
MeshDepthMaterial copy(Material source) {
super.copy(source);
depthPacking = source.depthPacking;
map = source.map;
alphaMap = source.alphaMap;
displacementMap = source.displacementMap;
displacementScale = source.displacementScale;
displacementBias = source.displacementBias;
wireframe = source.wireframe;
wireframeLinewidth = source.wireframeLinewidth;
return this;
}