Picking issue with custom shaders

From the forum: https://groups.google.com/d/msg/cesium-dev/25jPqr5ODg0/gYHeDX8uDQAJ

> A better gtTF with custom shader generation pipeline. Right now we have to paste our shaders (base64 encoded) into the glTF after it is converted. But doing so makes the models unpickable because Cesium does some string replacing code to add a picking mode to the shader, which is not compatible with our custom shader. Using czm_ automatic uniforms should be easy in this pipeline.

Start with the linked Cesium forum discussion and trace the glTF custom-shader generation and picking path described in the issue. Identify where Cesium performs shader string replacement and how automatic uniforms could fit the pipeline. Done means converted models using custom shaders remain pickable without manually editing base64-encoded shaders.

Written by the indexing model from the issue text.