message: gltfpack: Fix size_t -> int conversion warning on MSVC
  Since we don't want to merge image buffer views we should pass variant=-1.
diff: @@ -1688,7 +1688,7 @@ bool process(cgltf_data* data, std::vector<Mesh>& meshes, const Settings& settin
  const char* img = static_cast<const char*>(image.buffer_view->buffer->data) + image.buffer_view->offset;
  size_t size = image.buffer_view->size;
  - size_t view = getBufferView(views, BufferView::Kind_Image, i, 1, false);
  + size_t view = getBufferView(views, BufferView::Kind_Image, -1, 1, false);
  assert(views[view].data.empty());
  views[view].data.append(img, size);
  