gltf-pipeline fix for _BATCHID semantic

[lilleyse]

This is a fix for when a gltf with PBR uses the _BATCHID semantic or other underscored semantics. It only affected loading a gltf model, not a b3dm.

The gltf-pipeline PR is here with a couple more notes: CesiumGS/gltf-pipeline#340

Bug reported on the forum: https://groups.google.com/forum/#!topic/cesium-dev/qMKvqBUGsoE

[cesium-concierge]

@lilleyse, thanks for the pull request! Maintainers, we have a signed CLA from @lilleyse, so you can review this at any time.

⚠️ 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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[likangning93]

@lilleyse looks good! @pjcozzi merge when ready.

[pjcozzi]

@lilleyse please update CHANGES.md in master if warranted.

[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/qMKvqBUGsoE

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

The issue at #6036 has just been closed and may resolve your issue. Look for the change in the next stable release of Cesium or get it now in the master branch on GitHub https://github.com/AnalyticalGraphicsInc/cesium.

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