Fix corner clipping typos in layout-2020
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inner_radii() had a minus sign that shouldn't be there.
And padding_edge_clip() and content_edge_clip() clearly need to to use
the padding_rect() and content_rect() instead of border_rect.
Tests:
- css/css-backgrounds/background-clip-padding-box-with-border-radius.html
- css/css-backgrounds/background-rounded-image-clip.html
---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#29683 (GitHub issue number if applicable)
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- [X] There are tests for these changes OR
- [ ] These changes do not require tests because ___
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4339b3bab4