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/emath/src/vec2.rs

@@ -1,4 +1,4 @@
-use std::ops::{Add, AddAssign, Div, Mul, MulAssign, Neg, Sub, SubAssign};
+use std::ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Neg, Sub, SubAssign};
 
 /// A vector has a direction and length.
 /// A [`Vec2`] is often used to represent a size.
@@ -203,7 +203,8 @@ impl Vec2 {
     /// ```
     #[inline(always)]
     pub fn angled(angle: f32) -> Self {
-        vec2(angle.cos(), angle.sin())
+        let (sin, cos) = angle.sin_cos();
+        vec2(cos, sin)
     }
 
     #[must_use]
@@ -407,6 +408,14 @@ impl MulAssign<f32> for Vec2 {
     }
 }
 
+impl DivAssign<f32> for Vec2 {
+    #[inline(always)]
+    fn div_assign(&mut self, rhs: f32) {
+        self.x /= rhs;
+        self.y /= rhs;
+    }
+}
+
 impl Mul<f32> for Vec2 {
     type Output = Vec2;
 
@@ -470,4 +479,20 @@ fn test_vec2() {
     assert_eq!(Vec2::DOWN.angle(), 0.25 * TAU);
     almost_eq!(Vec2::LEFT.angle(), 0.50 * TAU);
     assert_eq!(Vec2::UP.angle(), -0.25 * TAU);
+
+    let mut assignment = vec2(1.0, 2.0);
+    assignment += vec2(3.0, 4.0);
+    assert_eq!(assignment, vec2(4.0, 6.0));
+
+    let mut assignment = vec2(4.0, 6.0);
+    assignment -= vec2(1.0, 2.0);
+    assert_eq!(assignment, vec2(3.0, 4.0));
+
+    let mut assignment = vec2(1.0, 2.0);
+    assignment *= 2.0;
+    assert_eq!(assignment, vec2(2.0, 4.0));
+
+    let mut assignment = vec2(2.0, 4.0);
+    assignment /= 2.0;
+    assert_eq!(assignment, vec2(1.0, 2.0));
 }