* 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.
249f8bcb93