Add skeleton import functionality to mesh_loader

[kar-re]

Description

This adds the functionality of importing an animated mesh to RViz using the same resource_retriever as in mesh_loader.

Due to the way Ogre3D loads skeleton meshes a developer can automatically import an animated mesh using the same loadMeshFromResource method already present, but with the added functionality of assigning animation states.
The method is mostly the same as loadMeshFromResource but with changes to load the .skeleton file as well into the default resource group.

Happy to accept any constructive critiques or suggestions on how to make it better!

Checklist

• If you are addressing rendering issues, please provide:
  • Images of both, broken and fixed renderings.
  • Source code to reproduce the issue, e.g. a YAML or rosbag file with a MarkerArray msg.
• If you are changing GUI, please include screenshots showing how things looked before and after.
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  Due to the lack of active maintainers, we cannot provide support for older release branches anymore.
• [Will do if accepted! ] Did you change how RViz works? Added new functionality? Do not forget to update the tutorials and/or documentation on the ROS wiki
• While waiting for someone to review your request, please consider reviewing another open pull request to support the maintainers of RViz. Refer to the RViz Wiki for reviewing guidelines.

[rhaschke]

I don't yet see the use of this PR. Could you explain more, please?
According to the OGRE doc, skeleton definitions are loaded on demand, especially when referenced by a Mesh. So, do we really need to explicitly load them?

How do you intend animating a mesh based on the skeleton? The SkeletonPtr isn't used yet.

[kar-re]

Hi @rhaschke!

Yes, sorry for not providing enough of a background.

So, just as you say Ogre3D loads the skeleton definition on demand when being referenced by a mesh. Unfortunately due to the package://-URI Ogre3D has trouble autoloading the skeleton, and this PR remedies that issue by loading the .skeleton file if present, and then Ogre3D can find it in the default resource group.

Let me show this more clearly by using an example:
Say a developer is following this tutorial for using animated meshes in Rviz, and the developer knows it's using Ogre3D for rendering:
https://wiki.ogre3d.org/Intermediate+Tutorial+1

Then the developer will be met with the following error:
[ WARN] [1629966114.309848800]: OGRE EXCEPTION(6:FileNotFoundException): Cannot locate resource XXXX.skeleton in resource group General or any other group. in ResourceGroupManager::openResource at /build/ogre-1.9-kiU5_5/ogre-1.9-1.9.0+dfsg1/OgreMain/src/OgreResourceGroupManager.cpp (line 753)

And if the developer tries setting an animation state which isn't present Rviz will crash.
The effort to remedy this includes looking through the mesh_loader in order to see how RViz handles mesh loading, and replicating this in the same way that this PR does.

This PR therefore remedies this by checking if there's an .skeleton mesh present, loading that into the default resource group, and assigning it to the mesh.

So in summary, a small change, which makes the developer experience more seamless.

The reason I kept loadSkeletonFromResource public is to enable others to manually assign other skeletons to meshes, if that is wanted. For example the _notifySkeleton method.

[rhaschke]

Thanks for the clarification.
I guess you are using the skeleton feature from a custom display then as rviz doesn't provide support for this (yet).

[rhaschke]

Closing and reopening to re-trigger CI.

[kar-re]

Hi @rhaschke, just wanted to say thanks for implementing the code review change, and sorry for the late response. Please let me know if there's anything else I can do! :)