Use u8 in Rounding, and introduce Roundingf (#5563)

* Part of https://github.com/emilk/egui/issues/4019

As part of the work on adding a custom `Border` to everything, I want to
make sure that the size of `RectShape`, `Frame` and the future `Border`
is kept small (for performance reasons).

This PR changes the storage of the corner radius of rectangles from four
`f32` (one for each corner) into four `u8`. This mean the corner radius
can only be an integer in the range 0-255 (in ui points). This should be
enough for most people.

If you want to manipulate rounding using `f32`, there is a new
`Roundingf` to fill that niche.

crates/epaint/src/lib.rs

@@ -29,6 +29,8 @@ pub mod image;
 mod margin;
 mod mesh;
 pub mod mutex;
+mod rounding;
+mod roundingf;
 mod shadow;
 mod shape;
 pub mod shape_transform;
@@ -47,10 +49,12 @@ pub use self::{
     image::{ColorImage, FontImage, ImageData, ImageDelta},
     margin::Margin,
     mesh::{Mesh, Mesh16, Vertex},
+    rounding::Rounding,
+    roundingf::Roundingf,
     shadow::Shadow,
     shape::{
-        CircleShape, EllipseShape, PaintCallback, PaintCallbackInfo, PathShape, RectShape,
-        Rounding, Shape, TextShape,
+        CircleShape, EllipseShape, PaintCallback, PaintCallbackInfo, PathShape, RectShape, Shape,
+        TextShape,
     },
     stats::PaintStats,
     stroke::{PathStroke, Stroke, StrokeKind},