Add a Display impl for Vec2, Pos2, and Rect (#4428)

These three types currently have a `Debug` implementation that only ever
prints one decimal point. Sometimes it is useful to see more of the
number, or otherwise have specific formatting.

Add `Display` implementations that pass the format specification to the
member `f32`s for an easier way to control what is shown when debugging.

This allows doing e.g. `ui.label(format!("{:.4}", rect * scale))` which
currently prints zeroes if scale is small.

crates/emath/src/pos2.rs

@@ -1,3 +1,4 @@
+use std::fmt;
 use std::ops::{Add, AddAssign, Sub, SubAssign};
 
 use crate::*;
@@ -316,8 +317,19 @@ impl Div<f32> for Pos2 {
     }
 }
 
-impl std::fmt::Debug for Pos2 {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+impl fmt::Debug for Pos2 {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "[{:.1} {:.1}]", self.x, self.y)
     }
 }
+
+impl fmt::Display for Pos2 {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str("[")?;
+        self.x.fmt(f)?;
+        f.write_str(" ")?;
+        self.y.fmt(f)?;
+        f.write_str("]")?;
+        Ok(())
+    }
+}