* Clear some properties in `destroy`

* Update ParticleShader.js

The ` vec2 v = aVertexPosition;` is needless , 
the  `v` is a new vec2 , use `aVertexPosition` will puzzle users (the people that read the code).

The  ` if (color.a == 0.0) discard;` will take unexpected result.

* Update ParticleBuffer.js

* cosmetic