add common header

src/rtweekend.h

@@ -0,0 +1,29 @@
+#pragma once
+
+#include <cmath>
+#include <cstdlib>
+#include <limits>
+#include <memory>
+
+// usings
+
+using std::shared_ptr;
+using std::make_shared;
+using std::sqrt;
+
+// constants
+
+const double infinity = std::numeric_limits<double>::infinity();
+const double pi = 3.1415926535897932385;
+
+// utility functions
+
+inline double degrees_to_radians(double degrees)
+{
+    return degrees * pi / 180;
+}
+
+// common headers
+
+#include "ray.h"
+#include "vec3.h"