Function off default values

We should put this in a function that we can call to set default values.

  var fovy = 60;
  var aspect = width/height;
  var znear = 0.001;
  var zfar = 1000;

  var ymax = znear * Math.tan(fovy * Math.PI / 360.0);
  var ymin = -ymax;
  var xmin = ymin * aspect;
  var xmax = ymax * aspect;

  var left = xmin;
  var right = xmax;
  var top =  ymax;
  var bottom = ymin;

  var X = 2 * znear / (right - left);
  var Y = 2 * znear / (top - bottom);
  var A = (right + left) / (right - left);
  var B = (top + bottom) / (top - bottom);
  var C = -(zfar + znear) / (zfar - znear);
  var D = -2 * zfar * znear / (zfar - znear);