Fix : take rounding into account when using Slider::trailing_fill (…

…emilk#4308)

Handles `rounding` when doing trailing_fill on the rail of `Slider`.

We can see this by setting the rounding to around 8.0

```
        ui.visuals_mut().widgets.inactive.rounding = Rounding::same(8.0);
```



Before : There is a little bit of blue painted on the left end.


![20240404-2](https://github.com/emilk/egui/assets/127506429/aa70104c-0733-41c6-8e78-c3e69eb45204)

After : Fix


![20240404-3](https://github.com/emilk/egui/assets/127506429/c2452fcb-48fd-4b2a-9f1a-02a3bf763ed1)

crates/egui/src/widgets/slider.rs

@@ -686,12 +686,10 @@ impl<'a> Slider<'a> {
 
             let rail_radius = (spacing.slider_rail_height / 2.0).at_least(0.0);
             let rail_rect = self.rail_rect(rect, rail_radius);
+            let rounding = widget_visuals.inactive.rounding;
 
-            ui.painter().rect_filled(
-                rail_rect,
-                widget_visuals.inactive.rounding,
-                widget_visuals.inactive.bg_fill,
-            );
+            ui.painter()
+                .rect_filled(rail_rect, rounding, widget_visuals.inactive.bg_fill);
 
             let position_1d = self.position_from_value(value, position_range);
             let center = self.marker_center(position_1d, &rail_rect);
@@ -707,13 +705,17 @@ impl<'a> Slider<'a> {
 
                 // The trailing rect has to be drawn differently depending on the orientation.
                 match self.orientation {
-                    SliderOrientation::Vertical => trailing_rail_rect.min.y = center.y,
-                    SliderOrientation::Horizontal => trailing_rail_rect.max.x = center.x,
+                    SliderOrientation::Horizontal => {
+                        trailing_rail_rect.max.x = center.x + rounding.nw;
+                    }
+                    SliderOrientation::Vertical => {
+                        trailing_rail_rect.min.y = center.y - rounding.se;
+                    }
                 };
 
                 ui.painter().rect_filled(
                     trailing_rail_rect,
-                    widget_visuals.inactive.rounding,
+                    rounding,
                     ui.visuals().selection.bg_fill,
                 );
             }