Fix the missing index buffer offset for skinned mesh instance.

[HanetakaChou]

 // ...

 for (const auto& skinnedInstance : m_SceneGraph->GetSkinnedMeshInstances())
    {
        const auto& skinnedMesh = skinnedInstance->GetMesh();

        if (!skinnedMesh->buffers)
        {
            skinnedMesh->buffers = std::make_shared<BufferGroup>();

            uint32_t totalVertices = skinnedMesh->totalVertices;

            skinnedMesh->buffers->indexBuffer = skinnedInstance->GetPrototypeMesh()->buffers->indexBuffer;
            skinnedMesh->buffers->indexBufferDescriptor = skinnedInstance->GetPrototypeMesh()->buffers->indexBufferDescriptor;

 // ...

According to the above code within Scene::CreateMeshBuffers, the same index buffer may be shared by different skinned meshes.

Thus, the index offset should NOT be omitted when creating the skinned mesh.

[HanetakaChou]

I have read the CLA Document and I hereby sign the CLA

[HanetakaChou]

@apanteleev

[VinnyVicious]

Also need this. Thanks @HanetakaChou