Set the default color of geoms according to the MJCF spec

[azeey]

🦟 Bug fix

Summary

The default rgba value of a geom is "0.5 0.5 0.5 1" (https://mujoco.readthedocs.io/en/latest/XMLreference.html#body-geom) For the following MJCF file

<mujoco>
  <worldbody>
    <geom type="box" size="1 1 1"/>
  </worldbody>
</mujoco>

Mujoco displays:

Before this PR, gz-sim displays

After this PR, gz-sim displays

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[bot]]

Codecov Report

Merging #84 (2750fba) into main (8e199b5) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #84      +/-   ##
==========================================
+ Coverage   95.14%   95.16%   +0.01%     
==========================================
  Files          24       24              
  Lines        1092     1096       +4     
==========================================
+ Hits         1039     1043       +4     
  Misses         53       53              

Impacted Files	Coverage Δ
...ormat_mjcf/mjcf_to_sdformat/converters/material.py	95.45% <100.00%> (+0.45%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 8e199b5...2750fba. Read the comment docs.