vulkan: Initial Vulkan support!

This work was done by Jacob Lifshay and Mark Callow; I'm just merging it
into revision control.

include/SDL.h

@@ -56,6 +56,7 @@
 #include "SDL_timer.h"
 #include "SDL_version.h"
 #include "SDL_video.h"
+#include "SDL_vulkan.h"
 
 #include "begin_code.h"
 /* Set up for C function definitions, even when using C++ */