sample method
Writes the spawn position and unit emission direction for particle
index into storage.
Implementation
@override
void sample(ParticleStorage storage, int index) {
_box.sample(storage, index, _saltA, _tmp);
storage.posX[index] = _tmp.x;
storage.posY[index] = _tmp.y;
storage.posZ[index] = _tmp.z;
storage.velX[index] = direction.x;
storage.velY[index] = direction.y;
storage.velZ[index] = direction.z;
}