DepthCamera and RGBDCamera - optimize RGB point cloud connection

[iche033]

🦟 Bug fix

Summary

Related pull request: gazebosim/gz-rendering#965

• Updated DepthCameraSensor and RGBDCameraSensor to connect to the point cloud connection only when there is a subscriber.

To be used together with gazebosim/gz-rendering#965

Here are the results showing the time taken to perform a Render() operation by the depth camera in gz-rendering before and after the changes.

• blue line (before) - render time taken is consistent regardless of whether you subscribe to depth or point cloud data.
• red line (after) - I first subscribed to get depth data. After approx 120 iterations, I switched to get rgb point cloud data and you can see the jump in time taken - it still does better than before due to minor optimizations done in: Optimize Ogre2DepthCamera performance by skipping color target passes when there are no connections gz-rendering#965

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• 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 and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (d522779) 72.78% compared to head (f3808e4) 72.77%.

Files	Patch %	Lines
src/RgbdCameraSensor.cc	70.00%	3 Missing ⚠️
src/DepthCameraSensor.cc	85.71%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           gz-sensors8     #413      +/-   ##
===============================================
- Coverage        72.78%   72.77%   -0.02%     
===============================================
  Files               36       36              
  Lines             4972     4980       +8     
===============================================
+ Hits              3619     3624       +5     
- Misses            1353     1356       +3     

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[iche033]

merging this as it does not require an upstream release. For binary installs, the perf improvement will only kick in when a gz-rendering8 release is made