mix method
Interpolate between min and max with the amount of a using a linear
interpolation and store the values in result.
Implementation
static void mix(Vector3 min, Vector3 max, double a, Vector3 result) {
  result
    ..x = min.x + a * (max.x - min.x)
    ..y = min.y + a * (max.y - min.y)
    ..z = min.z + a * (max.z - min.z);
}