Fix Model crash that occurs when specular environment maps are unsupported

[j9liu]

Fixes #10858. To reiterate, many Sandcastles that used image-based lighting were crashing on mobile device browsers that don't support specular environment maps. In ImageBasedLightingStageFS, the function textureIBL would always expect model_iblReferenceframeMatrix to be declared even when specular environment maps were disabled. This PR addresses that bug.

[cesium-concierge]

Thanks for the pull request @j9liu!

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Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.

[j9liu]

@lilleyse can you review this when you get the chance?

[lilleyse]

Thanks @j9liu!