Change ui.disable() to modify opacity (#7282)

* Closes https://github.com/emilk/egui/pull/6765


Branched off of https://github.com/emilk/egui/pull/6765 by @tye-exe.

I needed to branch off to update snapshot images

---------

Co-authored-by: tye-exe <tye@mailbox.org>
Co-authored-by: Tye <131195812+tye-exe@users.noreply.github.com>

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