Investigate GLSL optimizer

[pjcozzi]

I added a new github label for stages in an optimization pipeline that would be part of the converter and standalone as we discussed.

This issue is to investigate integrating a GLSL optimizer - https://github.com/aras-p/glsl-optimizer

[pjcozzi]

@gbeatty looked into this shader optimizer for Cesium. For our shaders, which are mostly hand-coded, it did not improve rendering performance on desktop or Android (Tegra 3 and Mali-400 MP). It also had issues with initializing structs and created rendering artifacts in some shaders. However, it did improve compile times on both Android devices.

For glTF, this could be a useful option if the source shaders (from ColladaFX, for example) are not carefully written and to improve mobile compile times; however, we should not do this by default and I would consider this a low priority even post glTF 1.0.

[pjcozzi]

Moving to gltf-pipeline so it is useful to all exporters/converters, see CesiumGS/gltf-pipeline#1.