closed #5331: _particleIdx may not equal to _particleCount if developer ...

...invoke Particle::addParticle() at schedule.

If user use addParticle() at schedule, and if Particle::update priority is higher than the schedule, the added particle won't be initialed, and _particleIdx won't be updated, so _particleIdx may not equal to _particleCount at ParticleSystemQuad::draw().