Can't lightmap CSGShapes and generated MeshInstances (cube, sphere, capsule, etc.)

[Janders1800]

Godot version:
3.2.3 stable

OS/device including version:
Zorin OS 15

Issue description:
In the case of CSGShapes wen you check "Use in baked light" in the 'GeometryInstance' section it doesn't get lightmaped.
For generated MeshInstances (cube, sphere, capsule, etc.) you can't unwrap UV2 for lightmap.

Steps to reproduce:

• Create CSGShape or a generated MeshInstance
• Check "Use in baked light" in the 'GeometryInstance' section
• Bake Lightmap

Minimal reproduction project:

LightmapMeshes.zip

[Calinou]

This is fixed in 4.x by #93252 and #67975. #98297 further improves the in-editor documentation on this.