Fix OSG transparent object sorting

[jslee02]

Transparent objects sometimes cannot be seen through other transparent objects. This seems to happen because the objects are not properly grouped for z-depth sorting. This PR is to fix it referencing this post.

Current behavior:

New behavior with this fix:

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[codecov]

Codecov Report

Merging #1414 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1414      +/-   ##
==========================================
+ Coverage   57.29%   57.31%   +0.02%     
==========================================
  Files         367      367              
  Lines       27521    27521              
==========================================
+ Hits        15768    15774       +6     
+ Misses      11753    11747       -6

Impacted Files	Coverage Δ
dart/common/Uri.cpp	68.82% <ø> (ø)	⬆️
dart/dynamics/EulerJoint.cpp	73% <0%> (+3.68%)	⬆️