Vulkan fixes

Fixes two important bug fixes:

1. Compilation error with "std::string", possibly caused by "#pragma once"
2. Fix for NVidia GPUs. See below for details:

https://devtalk.nvidia.com/default/topic/1054240/vulkan/no-rendering-with-vk_khr_multiview-on-gtx-1080-for-gt-1-view-multiple-drivers-versions-/