Fix specular flickering in processModelMaterialsCommon

[lilleyse]

If a model uses the KHR_materials_common extension, sets its technique to "PHONG", but doesn't provide specular or shininess values in its material, default values are 0 are used. This causes something to go wrong in the shader math resulting in a flickery effect. This fix is just smarter about checking if the model really has specular.

This issue has come up on the forum a couple times now:
https://groups.google.com/forum/#!topic/cesium-dev/kR7u5R5wq5k
https://groups.google.com/forum/#!topic/cesium-dev/JtTnEncijn0

[cesium-concierge]

@lilleyse, thanks for the pull request!

I noticed that CHANGES.md has not been updated. If this change updates the public API in any way, fixes a bug, or makes any non-trivial update, please add a bullet point to CHANGES.md and comment on this pull request so we know it was updated. For more info, see the Pull Request Guidelines.

I noticed that a file in one of our ThirdParty folders (ThirdParty/, Source/ThirdParty/) has been added or modified. Please verify that it has a section in LICENSE.md and that its license information is up to date with this new version. Once you do, please confirm by commenting on this pull request.

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[pjcozzi]

Did you also update gltf-pipeline?

Please update CHANGES.md and merge.

[cesium-concierge]

Congratulations on closing the issue! I found these Cesium forum links in the comments above:

https://groups.google.com/forum/#!topic/cesium-dev/kR7u5R5wq5k
https://groups.google.com/forum/#!topic/cesium-dev/JtTnEncijn0

If this issue affects any of these threads, please post a comment like the following:

The issue at #5773 has just been closed and may resolve your issue. Look for the change in thenext stable release of Cesium or get it now in the master branch on GitHub https://github.com/AnalyticalGraphicsInc/cesium.

I am a bot who helps you make Cesium awesome! Thanks again.