Added an osgDB::ReadWriteMutex to help manage the ability to have serialize

write to objects but allow multiple threads to read at once in a read only way.

src/osgParticle/ParticleSystem.cpp

@@ -91,6 +91,8 @@ void osgParticle::ParticleSystem::update(double dt)
 
 void osgParticle::ParticleSystem::drawImplementation(osg::State& state) const
 {
+    osgDB::ScopedReadLock lock(_readWriteMutex);
+
     // update the frame count, so other objects can detect when
     // this particle system is culled
     _last_frame = state.getFrameStamp()->getFrameNumber();