Fix bug in StaticGroundGeometryPerMaterialBatch

[impactblue573]

• Fix a bug in StaticGroundGeometryPerMaterialBatch where invalidated items are removed, causing a local array length to no longer be correct. This leads to calling functions on undefined causing a Cesium render error.

Sandcastle Example of crash

[cesium-concierge]

Thank you so much for the pull request @impactblue573! I noticed this is your first pull request and I wanted to say welcome to the Cesium community!

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[hpinkos]

Thanks for the PR @impactblue573! It looks like this might be a problem in some of the other static batches: https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/DataSources/StaticGeometryPerMaterialBatch.js#L376

Can you check all of those and make the same fix if necessary?

Also, do you have a code example to reproduce the crash this fixes?

[impactblue573]

@hpinkos Updated those other files, and also included a Sandcastle example of the crash. Thanks!

[hpinkos]

👍 thanks @impactblue573, works great!

[hpinkos]

I'll update CHANGES.md in master