Speed up fractal_clock painting (#152)

clip unwatchable line before drawing

emath/src/rect.rs

@@ -91,6 +91,14 @@ impl Rect {
             max: pos2(*x_range.end(), *y_range.end()),
         }
     }
+    pub fn from_two_pos(a: Pos2, b: Pos2) -> Self {
+        let (left, right) = if a.x > b.x { (b.x, a.x) } else { (a.x, b.x) };
+        let (top, bottom) = if a.y > b.y { (b.y, a.y) } else { (a.y, b.y) };
+        Rect {
+            min: pos2(left, top),
+            max: pos2(right, bottom),
+        }
+    }
 
     /// Expand by this much in each direction, keeping the center
     #[must_use]