Fix upside down slider in the vertical orientation (#3424)

crates/egui/src/widgets/slider.rs

@@ -703,7 +703,9 @@ impl<'a> Slider<'a> {
         let handle_radius = self.handle_radius(rect);
         match self.orientation {
             SliderOrientation::Horizontal => rect.x_range().shrink(handle_radius),
-            SliderOrientation::Vertical => rect.y_range().shrink(handle_radius),
+            // The vertical case has to be flipped because the largest slider value maps to the
+            // lowest y value (which is at the top)
+            SliderOrientation::Vertical => rect.y_range().shrink(handle_radius).flip(),
         }
     }
 

crates/emath/src/range.rs

@@ -97,6 +97,16 @@ impl Rangef {
         }
     }
 
+    /// Flip the min and the max
+    #[inline]
+    #[must_use]
+    pub fn flip(self) -> Self {
+        Self {
+            min: self.max,
+            max: self.min,
+        }
+    }
+
     /// The overlap of two ranges, i.e. the range that is contained by both.
     ///
     /// If the ranges do not overlap, returns a range with `span() < 0.0`.