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 {ReadonlyVec2} a the first operand
@param {ReadonlyVec2} b the second operand
@param {Number} t interpolation amount, in the range [0-1], between the two inputs
@returns {vec2} out