Change type of corner radius to f32 inside RectShape

[BeastLe9enD]

Makes it possible to draw rect shapes with better precision. For example, in my case, I am drawing a rect with scaling via TSTransform, and I want the rect to preserve the same corner radius at different scale resolutions. When using CornerRadius, it obviously doesn't have enough precision. I made the change with as few modifications as possible, only affecting a few parts. Of course, it's a breaking change, but since it's only inside the epaint part and not the egui part, I think it's acceptable.

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/7392-rectshapecornerradiusf32
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

[emilk]

So the rounding to closest integer is visible when you zoom in and out? Can you maybe show a video demonstrating this?

The reason we use u8 is to reduce the size of the RectShape (see the failing tests), but there are other solutions we could consider (like F16)