Import/export improvements

[andybak]

1. Adds an optional .glb export direct from the scene hierarchy using UnityGLTF. Brush stroke node names are changed but other than that the scene is exported as it exists in Unity at runtime
2. Choice of which export formats are used is now controlled from the Open Brush config file (see below for format. Defaults are as shown.
3. Platform configs still take priority (USD and FBX aren't supported on mobile etc) but experimental mode now has no effect
4. LATK support is no longer controlled from compiler flags or platform config

Example "Export" block from the config showing defaults (ExportBinaryFbx and ExportFbxVersion are existing keys)

    "Export": {
      "ExportBinaryFbx": true,
      "ExportFbxVersion": "FBX201400",
      "Formats": {
        "fbx": true,
        "glb": true,
        "newglb": false,
        "json": false,
        "latk": false,
        "obj": true,
        "stl": false,
        "usd": false,
        "wrl": false
      }
    }

5. Adds option to use UnityGLTF instead of GLTFast for the importer:

  "Import": {
    "UseUnityGltf": true
  }

6. Lights in GLTF files now import correctly. Previously they had several issues.
7. Lights in GLTF files now display a UI gizmo in the scene giving a sense of their direction and color. This will eventually be expanded to be a separate light gizmo

[mikeage]

The config file vs define stuff looks good; I have no opinion (or ability to form an opinion) on the actual UnityGLTF itself

[mikeage]

looks good from my POV, but I don't really have any substantive comments about UnityGLTF itself

[mikeage]

Also, prior to merging this, delete the Package cache (as well as the Library caches). I just realized I might have implied only the library caches on discord.

[mikeage]

Assets/Scripts/ModelCatalog.cs(255,33): error CS7036: There is no argument given that corresponds to the required formal parameter 'layerIds' of 'ModelWidget.CreateModelsFromRelativePath(string, string[], TrTransform[], TrTransform[], bool[], uint[], int[])'
Assets/Scripts/ModelCatalog.cs(261,33): error CS7036: There is no argument given that corresponds to the required formal parameter 'layerIds' of 'ModelWidget.CreateModelsFromRelativePath(string, string[], TrTransform[], TrTransform[], bool[], uint[], int[])'

Side effect of #689