Round widgets to integer coordinates

[emilk]

• Part of Round widget coordinates to integers #5163

After implementing this, I think this is taking it too far.

I think instead we should round to something like 1/16th. That will remove precision issues, but still give us very accurate layouts, smooth scrolling, etc.

To implement this we could do something like

const ROUND_FACTOR: f32 = 1.0 / 16.0

trait GuiRounding {
    /// Round to a multiple of [`ROUND_FACTOR`], to remove precision problems.
    fn round_gui(&self) -> Self;
}

impl GuiRounding for f32 { … }
impl GuiRounding for Vec2 { … }
impl GuiRounding for Pos2 { … }
impl GuiRounding for Rect { … }

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/5197-emilk/round-widgets
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