Add screenshot support for eframe web

[lucasmerlin]

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 Implement ViewportCommand::Screenshot on web #5425
• I have followed the instructions in the PR template

Bildschirmaufnahme.2024-12-05.um.15.27.52.mov

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/5438-lucaseframe-web-screenshots
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

[lucasmerlin]

I put this back to draft because surface.get_capabilities(&render_state.adapter).usages.contains(wgpu::TextureUsages::COPY_DST); doesn't really work as expected, @Wumpf will open a wgpu issue about it.

I'll need to update this to render the texture to the surface via a triangle instead of using copy_texture_to_texture, so we can properly support all platforms. I'll also do this for the native implementation, since there screenshots also don't work with the wgpu gl backend.

[Wumpf]

Related wgpu things:

• fix doc string for SurfaceConfiguration::usage gfx-rs/wgpu#6668
• Surface usage incorrectly always reported as wgt::TextureUsages::RENDER_ATTACHMENT for WebGPU gfx-rs/wgpu#6667

... but really the problem is that COPY_DST usage for surfaces is not portable and potentially slow

[Wumpf]

nice!
PR description is now outdated.
Comments are mostly nits about styling and efficiency (if anything the encoder one bugs me most actually) but what I'd like to insist on is refactoring/removing the blocking path - this code dupl doesn't really make sense and I don't think we should have a blocking path in the first place.

[Wumpf]

as noted above, it would be faster & easier to force the temp texture to be rgba

[lucasmerlin]

@Wumpf I've implemented everything you asked for (except for the texture_format changes, as discussed)

[Wumpf]

thank you so much for addressing all those! 🚢