glTF 2.0 - Brdf look up texture fails on Samsung Galaxy S6 edge

On a Samsung Galaxy S6 edge `BrdfLutGenerator.js` (called by `Model`) is crashing Cesium. It's hard to tell if this is a shader compilation bug or something with the draw command execution - but it all looks ok at a quick glance.

https://groups.google.com/forum/#!topic/cesium-dev/BbHs7YRg1DA

Start with BrdfLutGenerator.js and its call path from Model, then reproduce the glTF 2.0 case on a Samsung Galaxy S6 edge. Use the linked Cesium discussion for device-specific context and determine whether the failure occurs during shader compilation or draw command execution. Done means the BRDF lookup texture no longer crashes Cesium on that device.

Written by the indexing model from the issue text.