issue #3990: setTotalParticles works as expected. _emissionRate has to be updated in setTotalParticles.

cocos/2d/CCParticleSystemQuad.cpp

@@ -527,6 +527,7 @@ void ParticleSystemQuad::setTotalParticles(int tp)
         _totalParticles = tp;
     }
     
+    _emissionRate = _totalParticles / _life;
     resetSystem();
 }
 

tests/test-cpp/Classes/ParticleTest/ParticleTest.cpp

@@ -1905,7 +1905,6 @@ void Issue3990::onEnter()
     
     _emitter->setPositionType(ParticleSystem::PositionType::GROUPED);
     _emitter->setTotalParticles(1000);
-    _emitter->setLife(20);
     
     _emitter->setPosition(VisibleRect::center());