Fix visual glitch on the right side of highly rounded rectangles (#4244)

* Part of #4238

When one side of a rectangle is all rounding we need to take care not to
produce duplicated vertices in the rectangle path generator.

The old code only handled three sides, but forgot the last side (the
right side).
The new code handles the right side, and also handles the other sides
more robustly (with a floating point eps) and efficiently (in a single
pass).

The glitch was most notable in shadows with a high blur width.

Examples of the glitch:

<img width="203" alt="Screenshot 2024-03-26 at 20 15 38"
src="https://github.com/emilk/egui/assets/1148717/dc1c0a06-35f0-4fda-a011-0e37d18454a0">

<img width="220" alt="Screenshot 2024-03-27 at 09 48 48"
src="https://github.com/emilk/egui/assets/1148717/c278b28e-c3f9-4c82-ba20-0480621efd2f">

<img width="33" alt="Screenshot 2024-03-27 at 09 49 21"
src="https://github.com/emilk/egui/assets/1148717/379ddf77-6590-4444-9c2e-67ab1e071f0f">

crates/epaint/src/tessellator.rs

@@ -520,7 +520,7 @@ pub mod path {
         let min = rect.min;
         let max = rect.max;
 
-        let r = clamp_radius(rounding, rect);
+        let r = clamp_rounding(rounding, rect);
 
         if r == Rounding::ZERO {
             let min = rect.min;
@@ -531,11 +531,33 @@ pub mod path {
             path.push(pos2(max.x, max.y)); // right bottom
             path.push(pos2(min.x, max.y)); // left bottom
         } else {
-            add_circle_quadrant(path, pos2(max.x - r.se, max.y - r.se), r.se, 0.0);
-            add_circle_quadrant(path, pos2(min.x + r.sw, max.y - r.sw), r.sw, 1.0);
-            add_circle_quadrant(path, pos2(min.x + r.nw, min.y + r.nw), r.nw, 2.0);
-            add_circle_quadrant(path, pos2(max.x - r.ne, min.y + r.ne), r.ne, 3.0);
-            path.dedup(); // We get duplicates for thin rectangles, producing visual artifats
+            // We need to avoid duplicated vertices, because that leads to visual artifacts later.
+            // Duplicated vertices can happen when one side is all rounding, with no straight edge between.
+            let eps = f32::EPSILON * rect.size().max_elem();
+
+            add_circle_quadrant(path, pos2(max.x - r.se, max.y - r.se), r.se, 0.0); // south east
+
+            if rect.width() <= r.se + r.sw + eps {
+                path.pop(); // avoid duplicated vertex
+            }
+
+            add_circle_quadrant(path, pos2(min.x + r.sw, max.y - r.sw), r.sw, 1.0); // south west
+
+            if rect.height() <= r.sw + r.nw + eps {
+                path.pop(); // avoid duplicated vertex
+            }
+
+            add_circle_quadrant(path, pos2(min.x + r.nw, min.y + r.nw), r.nw, 2.0); // north west
+
+            if rect.width() <= r.nw + r.ne + eps {
+                path.pop(); // avoid duplicated vertex
+            }
+
+            add_circle_quadrant(path, pos2(max.x - r.ne, min.y + r.ne), r.ne, 3.0); // north east
+
+            if rect.height() <= r.ne + r.se + eps {
+                path.pop(); // avoid duplicated vertex
+            }
         }
     }
 
@@ -589,7 +611,7 @@ pub mod path {
     }
 
     // Ensures the radius of each corner is within a valid range
-    fn clamp_radius(rounding: Rounding, rect: Rect) -> Rounding {
+    fn clamp_rounding(rounding: Rounding, rect: Rect) -> Rounding {
         let half_width = rect.width() * 0.5;
         let half_height = rect.height() * 0.5;
         let max_cr = half_width.min(half_height);