SDL_Vulkan_LoadLibrary may fail in sandboxed Mac apps

On MacOS, calling SDL_Vulkan_LoadLibrary(NULL) is supposed to look for libvulkan or libMoltenVK.

However, if an app is sandboxed, it appears that this lookup is done in ~/Library/Containers/your.app.id/Data rather than /path/to/App/Contents/MacOS, which will cause Vulkan loading to fail.

