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
```
Adds support for animated WebP images. Used the already existing GIF
implementation as a template for most of it.
* [x] I have followed the instructions in the PR template
---------
Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>