Fix webgl errors when rendering over 64k billboards

[lilleyse]

Fixes #8315

Two separate bugs:

• When instancing is enabled - VertexArrayFacade breaks up vertex arrays where the number of vertices is greater than the ushort limit of 65536. This breaking up step shouldn't happen when doing instancing.
• When instancing is disabled - 9100963 broke non-instanced billboard rendering because the vertex array was no longer aligned to every 4 vertices.

Cesium should now be able to render large amounts of billboards without WebGL errors or artifacts whether the browser supports ANGLE_instanced_arrays or not.

[cesium-concierge]

Thanks for the pull request @lilleyse!

• ✔️ Signed CLA found.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[mramato]

Awesome, I wonder if there are any other random invalid GL type errors we've seen on Linux that will now go away.

[mramato]

Can confirm the referenced bug is fixed and things are working fine in both Linux and older browsers like IE11.