Add mathematical modulo helper function Mod(a, b)

planet/System.ocg/Math.c

@@ -160,4 +160,10 @@ global func GetSurfaceVector(int x, int y)
 	}
 	
 	return normal;
-}
+}
+
+// Proper modulo operation.
+global func Mod(a, b)
+{
+	return (a % b + b) % b;
+}