Minor optimization to transform control tool

[iche033]

🦟 Bug fix

Summary

While working on gazebosim/gz-rendering#800, I noticed that the transform control tool does unnecessary ray queries every frame if the user mouse click does not register any models. This is because the mouseDirtry variable never gets reset and hence every frame the transform control tool will continue to try and handle the same mouse event and perform ray queries.

This PR refactors the mouse event handle logic to its own function and resets the variable mouseDirtry.

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

Merging #1854 (50422fd) into ign-gazebo6 (68008d2) will increase coverage by 0.00%.
The diff coverage is 35.71%.

@@             Coverage Diff              @@
##           ign-gazebo6    #1854   +/-   ##
============================================
  Coverage        64.80%   64.80%           
============================================
  Files              322      322           
  Lines            26389    26391    +2     
============================================
+ Hits             17101    17103    +2     
  Misses            9288     9288           

Impacted Files	Coverage Δ
.../gui/plugins/transform_control/TransformControl.cc	7.36% <0.00%> (+0.01%)	⬆️
...rc/systems/ackermann_steering/AckermannSteering.cc	85.50% <71.42%> (-0.17%)	⬇️

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