Fix 3D Tiles Performance Regression

[lilleyse]

Fixes #7474

The problem was related to IBL and model shaders getting regenerated every frame. Fixed by adding equality checks in the setters.

[cesium-concierge]

Thanks for the pull request @lilleyse!

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[OmarShehata]

Just documenting, another failure on:

 Scene/Scene
    ✗ raises the cameraMoveEvent event when the camera stops moving
	Expected 0 to be 1.
	<Jasmine>
	@Specs/Scene/SceneSpec.js:1107:43
	window.it/<@Specs/customizeJasmine.js:76:30
	<Jasmine>

I restarted the build.

[mramato]

Looks good to me, @bagnell can you merge if happy?