Wrap glTF scripted importers with !MRTK_GLTF_IMPORTER_OFF

This enables them to be turned off if they collide with other importers like TriLib

Author: keveleigh

Assets/MRTK/Core/Utilities/Gltf/Serialization/Importers/GlbAssetImporter.cs

@@ -9,12 +9,14 @@
 
 namespace Microsoft.MixedReality.Toolkit.Utilities.Gltf.Serialization.Editor
 {
+#if !MRTK_GLTF_IMPORTER_OFF
     [ScriptedImporter(1, "glb")]
+#endif // !MRTK_GLTF_IMPORTER_OFF
     public class GlbAssetImporter : ScriptedImporter
     {
         public override void OnImportAsset(AssetImportContext context)
         {
             GltfEditorImporter.OnImportGltfAsset(context);
         }
     }
-}
+}

Assets/MRTK/Core/Utilities/Gltf/Serialization/Importers/GltfAssetImporter.cs

@@ -9,12 +9,14 @@
 
 namespace Microsoft.MixedReality.Toolkit.Utilities.Gltf.Serialization.Editor
 {
+#if !MRTK_GLTF_IMPORTER_OFF
     [ScriptedImporter(1, "gltf")]
+#endif // !MRTK_GLTF_IMPORTER_OFF
     public class GltfAssetImporter : ScriptedImporter
     {
         public override void OnImportAsset(AssetImportContext context)
         {
             GltfEditorImporter.OnImportGltfAsset(context);
         }
     }
-}
+}