Stop the viewport being moved when in the alt buffer

[j4james]

Summary of the Pull Request

When TerminalDispatch was merged with AdaptDispatch in PR #13024,
that broke the Terminal's EraseAll operation in the alt buffer. The
problem was that the EraseAll implementation makes a call to
SetViewportPosition which wasn't taking the alt buffer into account,
and thus modified the main viewport instead.

This PR corrects that mistake. If we're in the alt buffer, the
SetViewportPosition method now does nothing, since the alt buffer
viewport should always be at 0,0.

References

This was a regression introduced in PR #13024.

PR Checklist

• Closes Resizing while alt buffer is active, then disabling it, blows away renderer #13038
• CLA signed.
• Tests added/passed
• Documentation updated.
• Schema updated.
• I've discussed this with core contributors already. Issue number where discussion took place: Resizing while alt buffer is active, then disabling it, blows away renderer #13038

Validation Steps Performed

I've confirmed that the test case reported in issue #13038 is no longer
failing. I've also made sure the ED 2 and ED 3 sequences are still
working correctly in the main buffer.

[DHowett]

Thanks for the quick fix!

[j4james]

I'm sorry for breaking it in the first place!

[ghost]

Hello @zadjii-msft!

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[ghost]

🎉Windows Terminal Preview v1.14.143 has been released which incorporates this pull request.:tada:

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