Fix glTF metalness and roughness map orientation

[iche033]

🦟 Bug fix

Summary

glb file packs the metalness and roughness maps (grayscale) in a single texture (in the R and G channels) and our Assimp loader splits them out into individual textures for use in gz-rendering. However, the texture data are written out incorrectly (rotated by 90 degrees). This PR fixes that logic.

For testing, I'm using the water bottle glb file from: https://github.com/KhronosGroup/glTF-Sample-Models/tree/master/2.0/WaterBottle. You can see a screenshot of what the water bottle supposed to look like in that page.

Here're screenshots in gazebo showing before and after the fix:

Before:

After:

Side note, with default lighting params in Gazebo, the bottle appears a lot darker than expected . In the screenshot above, I had to change the dir light diffuse and specular colors to [1.0 1.0 1.0] and intensity to 2.0 to get the water bottle to be brighter

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 #532 (98ec53e) into gz-common5 (6e2444f) will not change coverage.
The diff coverage is 0.00%.

❗ Current head 98ec53e differs from pull request most recent head ade65d4. Consider uploading reports for the commit ade65d4 to get more accurate results

@@             Coverage Diff             @@
##           gz-common5     #532   +/-   ##
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  Coverage       83.65%   83.65%           
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  Files              90       90           
  Lines           10249    10249           
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  Hits             8574     8574           
  Misses           1675     1675           

Files Changed	Coverage Δ
graphics/src/AssimpLoader.cc	89.21% <0.00%> (ø)