egui: Fixed the incorrect display of the Window frame with a wide border or large rounding (#4032)

Currently, the Window frame is displayed incorrectly when using a wide
border or large rounding.

* Closes #3806
* Closes #4024 
* Closes #4025 

* Screencast of egui demo app (emilk:master)


[window-frame-bug.webm](https://github.com/emilk/egui/assets/1274171/391f67fa-ae6f-445a-8c64-1bb575770127)

* Screencast of egui demo app (varphone:hotfix/window-custom-frame)


[window-frame-fixed.webm](https://github.com/emilk/egui/assets/1274171/1953124e-9f7a-4c2d-9024-5d2eece6b87c)

crates/epaint/src/shape.rs

@@ -755,6 +755,130 @@ impl Rounding {
     }
 }
 
+impl std::ops::Add for Rounding {
+    type Output = Self;
+    #[inline]
+    fn add(self, rhs: Self) -> Self {
+        Self {
+            nw: self.nw + rhs.nw,
+            ne: self.ne + rhs.ne,
+            sw: self.sw + rhs.sw,
+            se: self.se + rhs.se,
+        }
+    }
+}
+
+impl std::ops::AddAssign for Rounding {
+    #[inline]
+    fn add_assign(&mut self, rhs: Self) {
+        *self = Self {
+            nw: self.nw + rhs.nw,
+            ne: self.ne + rhs.ne,
+            sw: self.sw + rhs.sw,
+            se: self.se + rhs.se,
+        };
+    }
+}
+
+impl std::ops::AddAssign<f32> for Rounding {
+    #[inline]
+    fn add_assign(&mut self, rhs: f32) {
+        *self = Self {
+            nw: self.nw + rhs,
+            ne: self.ne + rhs,
+            sw: self.sw + rhs,
+            se: self.se + rhs,
+        };
+    }
+}
+
+impl std::ops::Sub for Rounding {
+    type Output = Self;
+    #[inline]
+    fn sub(self, rhs: Self) -> Self {
+        Self {
+            nw: self.nw - rhs.nw,
+            ne: self.ne - rhs.ne,
+            sw: self.sw - rhs.sw,
+            se: self.se - rhs.se,
+        }
+    }
+}
+
+impl std::ops::SubAssign for Rounding {
+    #[inline]
+    fn sub_assign(&mut self, rhs: Self) {
+        *self = Self {
+            nw: self.nw - rhs.nw,
+            ne: self.ne - rhs.ne,
+            sw: self.sw - rhs.sw,
+            se: self.se - rhs.se,
+        };
+    }
+}
+
+impl std::ops::SubAssign<f32> for Rounding {
+    #[inline]
+    fn sub_assign(&mut self, rhs: f32) {
+        *self = Self {
+            nw: self.nw - rhs,
+            ne: self.ne - rhs,
+            sw: self.sw - rhs,
+            se: self.se - rhs,
+        };
+    }
+}
+
+impl std::ops::Div<f32> for Rounding {
+    type Output = Self;
+    #[inline]
+    fn div(self, rhs: f32) -> Self {
+        Self {
+            nw: self.nw / rhs,
+            ne: self.ne / rhs,
+            sw: self.sw / rhs,
+            se: self.se / rhs,
+        }
+    }
+}
+
+impl std::ops::DivAssign<f32> for Rounding {
+    #[inline]
+    fn div_assign(&mut self, rhs: f32) {
+        *self = Self {
+            nw: self.nw / rhs,
+            ne: self.ne / rhs,
+            sw: self.sw / rhs,
+            se: self.se / rhs,
+        };
+    }
+}
+
+impl std::ops::Mul<f32> for Rounding {
+    type Output = Self;
+    #[inline]
+    fn mul(self, rhs: f32) -> Self {
+        Self {
+            nw: self.nw * rhs,
+            ne: self.ne * rhs,
+            sw: self.sw * rhs,
+            se: self.se * rhs,
+        }
+    }
+}
+
+impl std::ops::MulAssign<f32> for Rounding {
+    #[inline]
+    fn mul_assign(&mut self, rhs: f32) {
+        *self = Self {
+            nw: self.nw * rhs,
+            ne: self.ne * rhs,
+            sw: self.sw * rhs,
+            se: self.se * rhs,
+        };
+    }
+}
+
 // ----------------------------------------------------------------------------
 
 /// How to paint some text on screen.