Fixed crash when creating plane with translation and normal on the Z axis

[IanLilleyT]

Sandcastle with crash
Sandcastle with fix

When creating the model matrix the code tries to generate an up vector that is different than the normal, and does this by setting the up vector to UNIT_Z. But if the normal itself is UNIT_Z, the problem is still there. So I added another check that sets it to UNIT_X if setting to UNIT_Z failed.

[cesium-concierge]

Thanks for the pull request @IanLilleyT!

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[hpinkos]

👍