2D mode rendering artefacts in iOS/Safari 15

The bug can be triggered by opening most sandcastle examples and switching to a 2D or columbus view.
Sometimes going to full screen makes the problem more obvious.

This is probably caused by the change of webGL backend that now uses Metal by default on iOS.

Sandcastle example:
https://sandcastle.cesium.com/gallery/Spheres%20and%20Ellipsoids.html
I think all sandcastle are impacted when switching to 2D mode

Browser:
Safari 15

Operating System:
iPadOS 15

Reproduce the issue with the Spheres and Ellipsoids Sandcastle example in Safari 15 on iPadOS 15, switching between 2D and Columbus views and checking fullscreen. Then trace the rendering path for those modes, keeping the reported Metal WebGL backend change as a hypothesis; done means the artefacts no longer appear across the affected Sandcastle examples.

Written by the indexing model from the issue text.