Improve texture filtering by doing it in gamma space (#7311)

* Closes https://github.com/emilk/egui/pull/5839

This makes some transparent images look a lot nicer when blended:


![image](https://github.com/user-attachments/assets/7f370aaf-886a-423c-8391-c378849b63ca)

Cursive text will also look nicer.

This unfortunately changes the contract of what
`register_native_texture` expects

---------

Co-authored-by: Adrian Blumer <blumer.adrian@gmail.com>

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