Keep diffuse intact so that shader generation for generateNormals works

[lilleyse]

Fix for a bug reported on the forum: https://groups.google.com/forum/#!topic/cesium-dev/b3iU1vlmaxo

• obj2gltf outputs KHR_materials_common which gltf-pipeline can generate shaders for
• If the model has no normals it sets the emission color instead of the diffuse color. This is needed for the constant lighting equation
• But if --generateNormals is set, gltf-pipeline will recreate the shaders again but this time expects the diffuse to exist.
• So instead this checks if normals will be regenerated before swapping the emission and diffuse.

I consider this just a bandaid for the time being and is more of a gltf-pipeline problem. Once gltf 2.0 is in full force this also won't be an issue.

[mramato]

Thanks @lilleyse! I confirmed this bug was in the last official release, so I updated changes.

We should do a release as soon as #85 is in.