Fix moving slider with arrow keys (#3354)

It would sometimes get stuck on the same value due to "smart-aim"

crates/egui/src/widgets/slider.rs

@@ -605,13 +605,14 @@ impl<'a> Slider<'a> {
         let kb_step = increment as f32 - decrement as f32;
 
         if kb_step != 0.0 {
+            let ui_point_per_step = 1.0; // move this many ui points for each kb_step
             let prev_value = self.get_value();
             let prev_position = self.position_from_value(prev_value, position_range);
-            let new_position = prev_position + kb_step;
+            let new_position = prev_position + ui_point_per_step * kb_step;
             let new_value = match self.step {
                 Some(step) => prev_value + (kb_step as f64 * step),
                 None if self.smart_aim => {
-                    let aim_radius = ui.input(|i| i.aim_radius());
+                    let aim_radius = 0.49 * ui_point_per_step; // Chosen so we don't include `prev_value` in the search.
                     emath::smart_aim::best_in_range_f64(
                         self.value_from_position(new_position - aim_radius, position_range),
                         self.value_from_position(new_position + aim_radius, position_range),