title: SDL_CreateWindow fails when SDL_WINDOW_VULKAN flag set
body: <p>when I try to run this code</p>
  
  <pre><code>if(SDL_Init(SDL_INIT_VIDEO) &lt; 0)
      printf("%s\n", SDL_GetError());
  if(!SDL_CreateWindow("test", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 800, 600, SDL_WINDOW_SHOWN | SDL_WINDOW_RESIZABLE | SDL_WINDOW_VULKAN))
      printf("%s\n", SDL_GetError());
  </code></pre>
  
  <p>after installing working gpu drivers and having linked vulkan,
  I get this output:</p>
  
  <p><strong>Vulkan support is either not configured in SDL or not available in video driver</strong></p>
  
  <p>I'm running on ubuntu with a Geforce GTX 660M + official drivers and SDL2 version 2.0.8. Seems like a bug in SDL, but I wanted to ask to make sure.</p>