Round the corners of the color picker (#8439)

The color picker's gradients and background checkers now follow the
style's corner radius, giving it a softer look.

This required changing the painting from being Mesh-based to being
texture-based

it's subtle with the default settings:

<img width="286" height="397" alt="Screenshot 2026-08-21 at 15 08 18"
src="https://github.com/user-attachments/assets/3a9d5289-71fb-4f34-8a20-1fe96891ba36"
/>


But you can [increase it](https://github.com/emilk/egui/pull/8445):


<img width="285" height="391" alt="Screenshot 2026-08-21 at 15 08 41"
src="https://github.com/user-attachments/assets/b0dfa7da-5b96-40e0-a80d-1da385d57a1a"
/>


### Before
for reference
<img width="286" height="392" alt="Screenshot 2026-08-21 at 19 08 27"
src="https://github.com/user-attachments/assets/7c754f36-795c-4381-9f9f-1fc8dd9a0264"
/>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

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