various fixes

class Viewport {

public:

  Viewport(

      int width,

      int height) :

      Viewport(width, height, std::tuple<int, int, int, int>(0,0,0,0)) {}

  Viewport(

      int width,

      int height,

      const std::tuple<int, int, int, int>& padding = {0,0,0,0}) :

      const std::tuple<int, int, int, int>& padding) :

      width_(width),

      height_(height),

      padding_(padding) {}