function getScreenConstraints(layout, width, height)
/**
 * Returns a set of constraints covering limits of screen
 * @param layout
 * @param width
 * @param height
 * @returns {Array}
 */
{
  var gap = 20;
  var size = layout._nodes.length;
  var topLeft = {x: 0, y: 0, fixed: true, index: size};
  var bottomRight = {x: width, y: height, fixed: true, index: size + 1};
  layout._nodes.push(topLeft);
  layout._nodes.push(bottomRight);
  var constraints = [];
  for (var i = 0; i < size; i++) {
    var index = layout._nodes[i].index;
    constraints.push({ axis: 'x', type: 'separation', left: topLeft.index, right: index, gap: gap });
    constraints.push({ axis: 'y', type: 'separation', left: topLeft.index, right: index, gap: gap });
    constraints.push({ axis: 'x', type: 'separation', left: index, right: bottomRight.index, gap: gap });
    constraints.push({ axis: 'y', type: 'separation', left: index, right: bottomRight.index, gap: gap });
  }
  return constraints;
}
Runs the layout computation on given nodes/links/groups
@returns Nothing