Fixes color picker hue drift at low alpha values (#8208)

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At low apha values, premultiplied colors lose precision.
This PR makes the color picker use unmultiplied colors internally.

Before:


https://github.com/user-attachments/assets/4617a355-daa9-4911-86e6-518ac6867014

After:


https://github.com/user-attachments/assets/d9681b01-50d8-418e-b5a5-79b4bd1bbddf

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