Fix missing tiles in VR

[bagnell]

The line here was the problem:
d681755#diff-6298b280e4e0977bedd881b65b769863L3232

The setup for each view for the eye was expecting the frustum to be a reference.

Fixes #6612.

[cesium-concierge]

Thanks for the pull request @bagnell!

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[lilleyse]

Thanks, this fixes it.