Extend AssimpLoader to parse material transmission factor

[iche033]

🎉 New feature

Summary

Adds basic support for material transmission extension.

Transmission can be used to simulate effects like tinting and refraction. Here we are currently just parsing the transmission factor property and modeling it as simple transparency setting.

Test it

Tested with the AnisotropyBarnLamp.glb model. The bulb part of the mesh is now transparent (previously just opaque white as shown in gazebosim/gz-rendering#976):

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• 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)
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.66%. Comparing base (3571cb4) to head (ade42a8).

Additional details and impacted files

@@             Coverage Diff             @@
##           gz-common5     #577   +/-   ##
===========================================
  Coverage       83.65%   83.66%           
===========================================
  Files              92       92           
  Lines           10287    10288    +1     
===========================================
+ Hits             8606     8607    +1     
  Misses           1681     1681           

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