Skybox flickers while viewports are synchronized

[pmconne]

Describe the bug
When two or more viewports are synchronized using TwoWayViewportSync, TwoWayFrustumSync, or connectViewports when navigating one of the viewports the skybox in the other viewport(s) flickers.

To Reproduce
Steps to reproduce the behavior:

1. Open a view in display-test-app with skybox enabled.
2. Clone the viewport using ctrl-.
3. Key in dta viewport sync
4. Navigate in one of the viewports

Expected behavior
The views in both viewports should be synchronized. They are, except the skybox flickers off and on in the inactive viewport.

Screenshots

Desktop (please complete the applicable information):

• OS: [e.g. iOS] irrelevant
• Browser [e.g. chrome, safari] irrelevant
• Version [e.g. 22] irrelevant
• iTwin.js Version 3.2.0

Additional context
This bug was introduced by #2661. The synchronization is done by cloning the ViewState, which causes the previous ViewState to detach from the Viewport and the new one to attach. The EnvironmentDecorations containing the skybox are deleted when detaching and recreated by the new ViewState when attaching. They load asynchronously so there is a brief period of time during which they cannot render until they finish loading.