Fixed "Scene/GroundPolylinePrimitive adds width instance attribute"

[IanLilleyT]

This test failed on mac when rendering a 0 width ground polyline, likely due to a small gpu/driver discrepancy that left a tiny sliver on the screen. So now this test uses an offset look position that a width 1 polyline is inside but a width 0 one is not.

[cesium-concierge]

Thanks for the pull request @IanLilleyT!

• ✔️ Signed CLA found.
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  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[likangning93]

Works (or fails gracefully) in IE11.

Unable to check because specs are broken in IE11 (#8083 (comment)), but it shouldn't be a problem because these specs aren't supported in IE11 and should just be returning without testing anyway.

[likangning93]

Thanks @IanLilleyT, looks to be working as advertised. I'll verify on another mac shortly.

[likangning93]

Confirmed working in Safari on a 2015 Macbook Pro (GT 750 model) running in Intel mode, although for some reason the og test also passes in this configuration. It might be an AMD specific thing. Anyway, thanks @lilleyse!