Fixed particle update bug where a ParticleSystem wouldn't start when loaded during the frame loop due to the _last_frame value not being set.

src/osgParticle/ParticleSystem.cpp

@@ -211,8 +211,6 @@ void osgParticle::ParticleSystem::update(double dt, osg::NodeVisitor& nv)
 
 void osgParticle::ParticleSystem::drawImplementation(osg::RenderInfo& renderInfo) const
 {
-    if (_particles.size() <= 0) return;
-
     ScopedReadLock lock(_readWriteMutex);
 
     osg::State& state = *renderInfo.getState();
@@ -221,6 +219,8 @@ void osgParticle::ParticleSystem::drawImplementation(osg::RenderInfo& renderInfo
     // this particle system is culled
     _last_frame = state.getFrameStamp()->getFrameNumber();
 
+    if (_particles.size() <= 0) return;
+
     // update the dirty flag of delta time, so next time a new request for delta time
     // will automatically cause recomputing
     _dirty_dt = true;