function lerp(out, a, b, t) {
  var ax = a[0],
      ay = a[1];
  out[0] = ax + t * (b[0] - ax);
  out[1] = ay + t * (b[1] - ay);
  return out;
}
Performs a linear interpolation between two vec2's

@param {vec2} out the receiving vector
@param {vec2} a the first operand
@param {vec2} b the second operand
@param {Number} t interpolation amount between the two inputs
@returns {vec2} out