Use scene name for texture name in assimp loader

[luca-della-vedova]

🦟 Bug fix

Summary

When importing assimp scenes, the name of the generated texture used the name of the root node as a prefix.
This worked well in simple GLB assets where there is only one node and the name would be its name but the moment a user was to import a scene with more than one node assimp would automatically create a new node and hardcode its name to ROOT as shown here.

This creates issues when running gz with ogre2 since we use the texture name to share textures between different assets, so different files would result in textures called ROOT_[...] and would cause the wrong texture to be displayed.

The fix is to use the scene name instead of the root node name (which was probably the right implementation in the first place).

Update: Found out why this wasn't the solution in the first place, CI fails on Focal since the field didn't exist in the assimp version released on 20.04:

  /github/workspace/graphics/src/AssimpLoader.cc:587:27: error: 'const struct aiScene' has no member named 'mName'
    587 |   return ToString(_scene->mName) + "_" + ToString(_mat->GetName()) +
        |                           ^~~~~
  make[2]: *** [graphics/src/CMakeFiles/gz-common5-graphics.dir/build.make:76: graphics/src/CMakeFiles/gz-common5-graphics.dir/AssimpLoader.cc.o] Error 1

Closing to retarget main.

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)
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