fix for issue #6612: Missing tiles in VR mode (regression w.r.t. Cesium 1.44)

[glathoud]

Two small commits to better support VR, in particular to avoid having missing tiles. See also #6612

[cesium-concierge]

Welcome to the Cesium community @glathoud!

Can you please send in a Contributor License Agreement (CLA) so that we can review and merge this pull request?

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[pjcozzi]

Thanks again @glathoud, we also received your CLA.

[hpinkos]

Thanks @glathoud! @bagnell could you review?

[bagnell]

Thanks for looking into this @glathoud. The problem was deeper in the engine where we change the frustum aspect ratios for the two eyes. Closing this in favor of #6798.

[glathoud]

@bagnell Thanks!