粒子的速度。
This velocity is used for effects that are based on physics. Examples of features that use this type of velocity are the Force module, Gravity, and Start Speed. This velocity is remembered across frames, and will be reapplied to the particle position of each frame.
如果渲染模式 设置为 ParticleSystemRenderMode.Stretch，则 ParticleSystemRenderer
也会使用该速度。
另请参阅：ParticleSystem.Particle.animatedVelocity、ParticleSystem.Particle.totalVelocity。