Feature: Add Vulkan graphics option using OpenXR create instance and device calls.

[jzulauf-lunarg]

OpenXR allows Vulkan VkInstance and VkDevice handles to be created by either native Vulkan vkCreateInstance and vkCreateDevice calls or using xrCreateVulkanInstanceKHR and xrCreateVulkanDeviceKHR calls.

OpenXR example code should show both possibilities (and allowing for testing using both).

@rokuz