Apply particle scattering effect to depth cameras

[iche033]

depends on #240 and #241

Summary

Particles emitted by the particle emitter used to be rendered as large 2D squares in the depth camera view due to the way they are represented in ogre (2d sprites / billboards). This PR applies "scattering effect" to the depth image output to make it appear as if the particle model is composed of a cluster of smaller particles. Gaussian noise is also applied to the particle depth values. Some scattering effect params are currently hardcoded since we cannot provide APIs to set them as it will break ABI.

Test it

You can test this using ign-gazebo by launching this sensor_particles.sdf world:

ign gazebo -v 4 sensor_particles.sdf

You should see:

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge

[codecov]

Codecov Report

Merging #251 (3f71f83) into ign-rendering4 (194bdf8) will increase coverage by 0.36%.
The diff coverage is 100.00%.

@@                Coverage Diff                 @@
##           ign-rendering4     #251      +/-   ##
==================================================
+ Coverage           52.92%   53.29%   +0.36%     
==================================================
  Files                 143      143              
  Lines               13566    13672     +106     
==================================================
+ Hits                 7180     7286     +106     
  Misses               6386     6386              

Impacted Files	Coverage Δ
ogre2/src/Ogre2Scene.cc	90.43% <ø> (ø)
ogre2/src/Ogre2DepthCamera.cc	94.50% <100.00%> (+0.87%)	⬆️
ogre2/src/Ogre2GpuRays.cc	94.75% <100.00%> (+0.51%)	⬆️
ogre2/src/Ogre2Visual.cc	85.14% <100.00%> (+0.30%)	⬆️

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[adlarkin]

I left a few minor comments. I tested the example world, and things seem to work for me. I had a hard time understanding a lot of what was going on in this PR though, so it might be worth having someone else review this as well to make sure I didn't miss anything.

[adlarkin]

As I mentioned before, my understanding of what's going on here isn't perfect, so if someone else wants to take a look as well, go for it, but this looks good to me.