Auto merge of #17395 - pyfisch:issue17387, r=emilio

Make background gradient cover padding

CSS-gradients should not only cover the content of an
element but also the padding (but not the border).

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Thanks to @atouchet for catching this.

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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 #17387 (github issue number if applicable).

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- [x] These changes do not require tests because gradients are hard to test automatically but you can use [this gist](https://gist.github.com/pyfisch/fa263c8dfc57e5812fe8a4869ad61513) as a manual test.

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