sample method
Writes the spawn position and unit emission direction for particle
index into storage.
Implementation
@override
void sample(ParticleStorage storage, int index) {
storage.posX[index] = 0.0;
storage.posY[index] = 0.0;
storage.posZ[index] = 0.0;
storage.velX[index] = direction.x;
storage.velY[index] = direction.y;
storage.velZ[index] = direction.z;
}