Implement XR_KHR_vulkan_enable2
Remaining issues:
- extension lists are already stored as long strings in comp_vk_glue.c, I didn't touch those and made a duplicate list of strings
- The spec doesn't say what should happen if vkGetInstanceProcAddr fails. Here we return XR_SUCCESS and set vulkanResult to VK_ERROR_INITIALIZATION_FAILED (vkGetInstanceProcAddr has no return value, the function pointer will just be NULL)