macOS test failures

I'm seeing two unit test failures on macOS as of 1.76. The failures go back at least as far as 1.70, I haven't checked earlier than that.

System: 2015 MacBook, AMD Radeon R9 M370X 2 GB, Big Sur (11.0.1)

- ~[Renderer/BuiltinFunctions has czm_decompressTextureCoordinates](http://localhost:8080/Specs/SpecRunner.html?spec=Renderer%2FBuiltinFunctions%20has%20czm_decompressTextureCoordinates)~
- [Scene/GeometryRenderingSpec SimplePolylineGeometry 2D](http://localhost:8080/Specs/SpecRunner.html?spec=Scene%2FGeometryRenderingSpec%20SimplePolylineGeometry%202D)

(The BuiltinFunctions specs were fixed in https://github.com/CesiumGS/cesium/pull/9821)

Start with the linked Scene/GeometryRenderingSpec SimplePolylineGeometry 2D test and reproduce it on the reported Big Sur system with the AMD Radeon R9 M370X. The BuiltinFunctions failure was fixed by pull request 9821, so focus on identifying the remaining rendering failure and confirm that the named spec passes on macOS.

Written by the indexing model from the issue text.