Support standardSurface shader

[alexcubor]

It is very important to use standardSurface instead of lambert, as this shader supports all the right texture maps and is becoming an industry standard. It is already successfully exported to USD for iOS environment. We would like the GLB exporter to be able to translate standardSurface into its own shader. Can we enable support for standardSurface?

[ziriax]

I'm currently employed by NVIDIA and I hardly have any free time for working on this, but pull requests are welcome. If I find spare time, I will be happy to look into this, but I can't give you any estimate.