Add emath::Rangef and helpers for splitting a Rect (#2978)

* emath: add Rangef helper

* emath: Add Rect splitting helper

* Add helper for interpolating Pos2

crates/emath/src/pos2.rs

@@ -188,6 +188,14 @@ impl Pos2 {
             y: self.y.clamp(min.y, max.y),
         }
     }
+
+    /// Linearly interpolate towards another point, so that `0.0 => self, 1.0 => other`.
+    pub fn lerp(&self, other: Pos2, t: f32) -> Pos2 {
+        Pos2 {
+            x: lerp(self.x..=other.x, t),
+            y: lerp(self.y..=other.y, t),
+        }
+    }
 }
 
 impl std::ops::Index<usize> for Pos2 {