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

epaint/Cargo.toml

@@ -43,3 +43,5 @@ single_threaded = ["atomic_refcell"]
 
 # Only needed if you plan to use the same fonts from multiple threads.
 multi_threaded = ["parking_lot"]
+
+mint = ["emath/mint"]