update method
Advances every live particle by dt seconds.
Implementation
@override
void update(ParticleStorage storage, double dt) {
final n = storage.aliveCount;
for (var i = 0; i < n; i++) {
final life = storage.lifetime[i];
final nAge = life > 0.0 ? storage.age[i] / life : 0.0;
color.sample(nAge, storage.random01[i], _tmp);
storage.colorR[i] = _tmp.x;
storage.colorG[i] = _tmp.y;
storage.colorB[i] = _tmp.z;
storage.colorA[i] = _tmp.w;
}
}