Don't crash when building lighting for a level with a Cesium3DTileset

[kring]

Fixes #1180

UnrealLightmass.exe is a separate process run via Swarm when building lighting. As reported in #1180, it (always? usually?) crashes when we attempt to build lighting (Build -> Build Lighting Only) for a level that has a Cesium3DTileset in it. Unfortunately it's build without any debug symbols, so I had to build it myself to debug it.

Having done that, I learned that the problem is that every FStaticMeshLightingMesh pointed to the same FStaticMesh instance, which really couldn't be right. And I learned that that was happening because, on import, all the static mesh references had the same GUID (all zeros!). Ok, so the problem seemed to be back in the Unreal Editor on the export side.

So, I traced through the code that exported the lightmass data, and yep, it was writing a GUID of all zeros. It got that GUID from a property on the UStaticMesh called LightingGuid. So how is that property set? I searched for it in the UE source code, and saw that things like DatasmithStaticMeshImporter call SetLightingGuid on the static just before they called InitResources. I added the same to our code that creates the static mesh in CesiumGltfComponent, and the lightmass crash went away.

I'll spare everyone the rant about how ridiculous it is that we have to call some random undocumented method on every static mesh we create in order to prevent UE's static lighting baking system from crashing. 🤷

[cesium-concierge]

Thanks for the pull request @kring!

• ✔️ Signed CLA found.

Reviewers, don't forget to make sure that:

• Cesium for Unreal Samples works.

[j9liu]

Thanks @kring , I confirmed this removes the crash when building lighting data. I'll merge once CI passes!

[kring]

This page was helpful to me in learning how to debug the lightmass executable:
https://ikrima.dev/ue4guide/graphics-development/lightmass-lightmapping/debugging-lightmass/

The really short version is you can run lightmassdebug in the console, and then launch a light build normally, and Swarm will wait for you to start UnrealLightmass.exe manually rather than launching it itself. So you can run your debug version of it in the Visual Studio debugger, and it will pick up and execute the waiting job.

Just putting this here for possible future reference.