I improved the material system, introducing the lighting stage. I removed the fragment stage and replaced it with color and lighting stage. The output color is calculated as the sum of the color stage and the lighting stage. The color stage has only one material node as input that is used to produce the output color for this stage.

The lighting stage wants more inputs, like ambient, diffuse (or albedo), specular, roughness, metalness, that are mixed in a physical based rendering (PBR). Each input is connected to one material node that can be the result of the operation between more material nodes, so every stage can have it's own textures or the math operation between more textures, uniforms and constants. In the video you can see a model with advanced materials imported to show the benefits of the last optimizations. In this case, the ambient stage is rendered correctly and mixed with the diffuse textures.