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/egui_demo_lib/src/demo/pan_zoom.rs

@@ -127,7 +127,7 @@ impl crate::View for PanZoom {
                 .show(ui.ctx(), |ui| {
                     ui.set_clip_rect(transform.inverse() * rect);
                     egui::Frame::default()
-                        .rounding(egui::Rounding::same(4.0))
+                        .rounding(egui::Rounding::same(4))
                         .inner_margin(egui::Margin::same(8.0))
                         .stroke(ui.ctx().style().visuals.window_stroke)
                         .fill(ui.style().visuals.panel_fill)