implement mint conversions (#352)

* Implement mint conversions

Implement conversions for [mint](https://docs.rs/mint) (math interoperability standard types).

- `impl {From, Into}<mint::Point2> for Pos2`
- `impl {From, Into}<mint::Vector2> for Vec2`

* Forward `mint` feature: egui -> epaint -> emath

emath/src/vec2.rs

@@ -81,6 +81,23 @@ impl From<&Vec2> for (f32, f32) {
     }
 }
 
+// ----------------------------------------------------------------------------
+// Mint compatibility and convenience conversions
+
+#[cfg(feature = "mint")]
+impl From<mint::Vector2<f32>> for Vec2 {
+    fn from(v: mint::Vector2<f32>) -> Self {
+        Self::new(v.x, v.y)
+    }
+}
+
+#[cfg(feature = "mint")]
+impl From<Vec2> for mint::Vector2<f32> {
+    fn from(v: Vec2) -> Self {
+        Self { x: v.x, y: v.y }
+    }
+}
+
 // ----------------------------------------------------------------------------
 
 impl Vec2 {