This implements web support for taking screenshots in an eframe app (and
adds a nice demo).
It also updates the native screenshot implementation to work with the
wgpu gl backend.
The wgpu implementation is quite different than the native one because
we can't block to wait for the screenshot result, so instead I use a
channel to pass the result to a future frame asynchronously.
* Closes <https://github.com/emilk/egui/issues/5425>
* [x] I have followed the instructions in the PR template
https://github.com/user-attachments/assets/67cad40b-0384-431d-96a3-075cc3cb98fb
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