function squaredDistance(a, b) {
  var x = b[0] - a[0],
      y = b[1] - a[1];
  return x * x + y * y;
}
Calculates the squared euclidian distance between two vec2's

@param {vec2} a the first operand
@param {vec2} b the second operand
@returns {Number} squared distance between a and b