Potential non-deterministic behavior in particle emitter (ogre2)

## Environment
* OS Version: Ubuntu Focal
* Source or binary build? Affects both source and binary in all versions

## Description

The particle emitter in ogre2 uses the Particle FX plugin. The plugin likely animates the particles using real time so the effect could be non-deterministic.

* Expected behavior: Speed of particles are dependent on user-specified time steps
* Actual behavior: Particles are animated using real time

## Steps to reproduce
1. Easier to reproduce using ign gazebo. Change `` in [particle_emitter2.sdf](https://github.com/ignitionrobotics/ign-gazebo/blob/ign-gazebo6/examples/worlds/particle_emitter2.sdf#L28) file to `0` so simulation runs as fast as possible.
2. Launch ign gazebo with the modified `particle_emitter2.sdf` world
3. See the particles flow upwards in regular speed when it should be flowing as fast as possible.

Start by reproducing the issue with the modified examples/worlds/particle_emitter2.sdf in ign gazebo, setting to 0. Then trace the OGRE2 particle emitter's use of the Particle FX plugin and how simulation time reaches it. Done means particle speed follows user-specified simulation time steps rather than wall-clock time, including when the real-time factor is zero.

Written by the indexing model from the issue text.