GLTF Loader: add vertex start position to the primitive

This addresses gltf file with mixed indicies/no indicies primitives

Author: cadams-dak

AssetLoader/interface/GLTFBuilder.hpp

@@ -444,6 +444,8 @@ Mesh* ModelBuilder::LoadMesh(const GltfModelType& GltfModel,
         if (GltfPrimitive.GetIndicesId() >= 0)
         {
             IndexCount = ConvertIndexData(GltfModel, GltfPrimitive.GetIndicesId(), VertexStart);
+            // For indexed primitives, the vertex offset is baked into the indices,
+            VertexStart = 0;
         }
 
         int MaterialId = GltfPrimitive.GetMaterialId();
@@ -461,6 +463,7 @@ Mesh* ModelBuilder::LoadMesh(const GltfModelType& GltfModel,
             IndexStart,
             IndexCount,
             VertexCount,
+            VertexStart,
             static_cast<Uint32>(MaterialId),
             PosMin,
             PosMax //

AssetLoader/interface/GLTFLoader.hpp

@@ -410,19 +410,22 @@ struct Primitive
     const Uint32 FirstIndex;
     const Uint32 IndexCount;
     const Uint32 VertexCount;
+    const Uint32 FirstVertex;
     const Uint32 MaterialId;
 
     const BoundBox BB;
 
     Primitive(Uint32        _FirstIndex,
               Uint32        _IndexCount,
               Uint32        _VertexCount,
+              Uint32        _FirstVertex,
               Uint32        _MaterialId,
               const float3& _BBMin,
               const float3& _BBMax) :
         FirstIndex{_FirstIndex},
         IndexCount{_IndexCount},
         VertexCount{_VertexCount},
+        FirstVertex{_FirstVertex},
         MaterialId{_MaterialId},
         BB{_BBMin, _BBMax}
     {