"Generate Lightmap UVs" does not expose options and does not override existing UV2

[hybridherbst]

Describe the bug 💬

Follow-up to #668. Unity's ModelImporter replaces UV2 when "Generate Lightmap UVs" is selected in the importer, UnityGLTF currently just uses the underlying Unwrapping.GenerateSecondaryUVSet() method, which seems to simply do nothing when there's existing UVs, so we probably need to delete the existing UVs first.

Additionally, it's probably a good idea to expose the parameters that go into that method, same as the FBX importer.

Steps to reproduce 🔢

1. Import file that has UV2
2. Enable "Generate Lightmap UVs"
3. Note no new UVs are generated, in contrast to Unity's model importer

Files to reproduce the issue ♻

See #668

Editor Version 🎲

2021.3

Render Pipeline and version

All

UnityGLTF Version

2.17.1

Operating System 👩‍💻

Windows, macOS, Linux

When does this problem happen?

• Editor Import
• Runtime Import
• Editor Export
• Runtime Export

Additional Info 📜

No response

Validations 🩹

• I have searched existing issues: no issue already exist that reports the same problem.
• I follow the Code of Conduct
• I provided a minimal reproducible example, including files when necessary.

[ludwigseibt]

I would like to note that the current behaviour, where existing lightmaps are preseverd, can also be useful at times. This should be handled with a checkbox "Override existing UV2". It should override existing by default, because that is the behaviour that is common in imported fbx files.

[hybridherbst]

I'll look at it. Note that we want to avoid adding options for each and every thing, as each additional exposed setting makes maintenance harder.

[ludwigseibt]

I understand completely. This option would be really helpful. Often times when working on larger environment assets, the generated lightmap UVs work fine except for a few objects. With the optional override disabled, we could selectively create lightmap UVs only for the objects that need manual adjustment. Another usecase would be delivering Work-In-Progress states of an environment where some objects dont have manual UVs yet.