Use impl Into<Stroke> as argument in a few more places (#3420)

* Functions that take Stroke were updated to take Into<Stroke> to make
them consistent with other Into<Stroke> parameters.
* Vec2 implements DivAssign<f32>, to make it consistent with already
implementing MulAssign<f32> and Div<f32>.
* Vec2::angled() uses sin_cos() rather than an individual sin() and
cos() call for an immeasurable but hypothetical performance improvement.
* Disable the lock_reentry_single_thread() mutex test. Lock()ing twice
on the same thread is not guaranteed to panic.

* Closes <https://github.com/emilk/egui/issues/3419>.

crates/epaint/src/mutex.rs

@@ -387,14 +387,6 @@ mod tests {
         let _b = two.lock();
     }
 
-    #[test]
-    #[should_panic]
-    fn lock_reentry_single_thread() {
-        let one = Mutex::new(());
-        let _a = one.lock();
-        let _a2 = one.lock(); // panics
-    }
-
     #[test]
     fn lock_multiple_threads() {
         use std::sync::Arc;