Fix UI scaling issues

[Weibye]

Objective

Fixes #3493
Adopted from and supersedes #3533

The issue was that extracted_uinode.rect is scaled according to the window's scale factor when extracted_uinode.atlas_size is None, but it's not scaled when extracted_uinode.atlas_size is Some.

Solution

I changed one condition to avoid relying on extracted_uinode.rect, extracted the window's scale factor into the render app, and applied it as necessary when calculating the clipped UVs.
To be honest, I can't properly explain why it works this way, and I'm not confident I covered all cases. In #3460, the similar fields extracted_sprite.rect and extracted_sprite.atlas_size are changed to cleanly separate what affects position/size and what affects UVs. I hope to port to UI the sprite improvements of that PR after it's accepted, but meanwhile this smaller PR can fix the issue.

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Changelog

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Migration Guide

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[Weibye]

Closing as obsolete. The problem has already been resolved