This fixes bugs related to how an `Image` follows the size of an SVG.
We track the "source size" of each image, i.e. the original width/height
of the SVG, which can be different from whatever it was rasterized as.
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
