Handle static RGB WebP images

[Aely0]

Current WebP loader assumes all WebP images to be RGBA, which is the case if the image is animated (that's what image crate assumes at least). Static images can instead choose to exclude its alpha channel, though it seems to be more of a default choice to include it, even if it's not being utilized. Currently, loading a static RGB WebP image will cause a panic when ColorImage::from_rgba_unmultiplied gets called in the loader

thread 'main' panicked at /home/aely/.cargo/git/checkouts/egui-226fc7cdd51201c1/f87219d/crates/epaint/src/image.rs:97:9:
assertion `left == right` failed
  left: 29184
 right: 21888

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/5586-webp-static-rgb
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

[emilk]

Nice