added function to construct a rounded rect

src/ScreensaverPlugin.cpp

@@ -73,3 +73,23 @@ double ScreensaverPlugin::random_between(double v1, double v2) {
     std::uniform_real_distribution<double> dist {v1, v2};
     return dist(_eng);
 }
+
+void ScreensaverPlugin::rounded_rect(const cairo_rectangle_t& rect, double radius) {
+    double x0 = rect.x;
+    double x1 = rect.x + radius;
+    double x3 = rect.x + rect.width;
+    double x2 = x3 - radius;
+    double y0 = rect.y;
+    double y3 = rect.y + rect.height;
+    double y1 = rect.y + radius;
+    double y2 = y3 - radius;
+    cairo_move_to(_c, x1, y0);
+    cairo_line_to(_c, x2, y0);
+    cairo_arc(_c, x2, y1, radius, 1.5 * M_PI, 0);
+    cairo_line_to(_c, x3, y2);
+    cairo_arc(_c, x2, y2, radius, 0, 0.5 * M_PI);
+    cairo_line_to(_c, x1, y3);
+    cairo_arc(_c, x1, y2, radius, 0.5 * M_PI, M_PI);
+    cairo_line_to(_c, x0, y1);
+    cairo_arc(_c, x1, y1, radius, M_PI, 1.5 * M_PI);
+}

src/ScreensaverPlugin.hpp

@@ -36,6 +36,7 @@ class ScreensaverPlugin {
         double random_y();
         double random01();
         double random_between(double v1, double v2);
+        void rounded_rect(const cairo_rectangle_t& rect, double radius);
 };
 
 #endif // _ScreensaverPlugin_H_