vulkan: SDL_Vulkan_GetInstanceExtensions should accept a NULL window.

Fixes Bugzilla #4235.

include/SDL_vulkan.h

@@ -135,7 +135,7 @@ extern DECLSPEC void SDLCALL SDL_Vulkan_UnloadLibrary(void);
  *  \brief Get the names of the Vulkan instance extensions needed to create
  *         a surface with \c SDL_Vulkan_CreateSurface().
  *
- *  \param [in]     window Window for which the required Vulkan instance
+ *  \param [in]     \c NULL or window Window for which the required Vulkan instance
  *                  extensions should be retrieved
  *  \param [in,out] count pointer to an \c unsigned related to the number of
  *                  required Vulkan instance extensions
@@ -153,6 +153,10 @@ extern DECLSPEC void SDLCALL SDL_Vulkan_UnloadLibrary(void);
  *  is smaller than the number of required extensions, \c SDL_FALSE will be
  *  returned instead of \c SDL_TRUE, to indicate that not all the required
  *  extensions were returned.
+ * 
+ *  \note If \c window is not NULL, it will be checked against its creation
+ *        flags to ensure that the Vulkan flag is present. This parameter
+ *        will be removed in a future major release.
  *
  *  \note The returned list of extensions will contain \c VK_KHR_surface
  *        and zero or more platform specific extensions
@@ -160,12 +164,13 @@ extern DECLSPEC void SDLCALL SDL_Vulkan_UnloadLibrary(void);
  *  \note The extension names queried here must be enabled when calling
  *        VkCreateInstance, otherwise surface creation will fail.
  *
- *  \note \c window should have been created with the \c SDL_WINDOW_VULKAN flag.
+ *  \note \c window should have been created with the \c SDL_WINDOW_VULKAN flag
+ *        or be \c NULL
  *
  *  \code
  *  unsigned int count;
  *  // get count of required extensions
- *  if(!SDL_Vulkan_GetInstanceExtensions(window, &count, NULL))
+ *  if(!SDL_Vulkan_GetInstanceExtensions(NULL, &count, NULL))
  *      handle_error();
  *
  *  static const char *const additionalExtensions[] =
@@ -179,7 +184,7 @@ extern DECLSPEC void SDLCALL SDL_Vulkan_UnloadLibrary(void);
  *      handle_error();
  *
  *  // get names of required extensions
- *  if(!SDL_Vulkan_GetInstanceExtensions(window, &count, names))
+ *  if(!SDL_Vulkan_GetInstanceExtensions(NULL, &count, names))
  *      handle_error();
  *
  *  // copy additional extensions after required extensions

src/video/SDL_video.c

@@ -4103,11 +4103,14 @@ void SDL_Vulkan_UnloadLibrary(void)
 
 SDL_bool SDL_Vulkan_GetInstanceExtensions(SDL_Window *window, unsigned *count, const char **names)
 {
-    CHECK_WINDOW_MAGIC(window, SDL_FALSE);
+    if (window) {
+        CHECK_WINDOW_MAGIC(window, SDL_FALSE);
 
-    if (!(window->flags & SDL_WINDOW_VULKAN)) {
-        SDL_SetError(NOT_A_VULKAN_WINDOW);
-        return SDL_FALSE;
+        if (!(window->flags & SDL_WINDOW_VULKAN))
+        {
+            SDL_SetError(NOT_A_VULKAN_WINDOW);
+            return SDL_FALSE;
+        }
     }
 
     if (!count) {

test/testvulkan.c

@@ -255,7 +255,7 @@ static void createInstance(void)
     appInfo.apiVersion = VK_API_VERSION_1_0;
     instanceCreateInfo.sType = VK_STRUCTURE_TYPE_INSTANCE_CREATE_INFO;
     instanceCreateInfo.pApplicationInfo = &appInfo;
-    if(!SDL_Vulkan_GetInstanceExtensions(state->windows[0], &extensionCount, NULL))
+    if(!SDL_Vulkan_GetInstanceExtensions(NULL, &extensionCount, NULL))
     {
         SDL_LogError(SDL_LOG_CATEGORY_APPLICATION,
                      "SDL_Vulkan_GetInstanceExtensions(): %s\n",
@@ -268,7 +268,7 @@ static void createInstance(void)
         SDL_OutOfMemory();
         quit(2);
     }
-    if(!SDL_Vulkan_GetInstanceExtensions(state->windows[0], &extensionCount, extensions))
+    if(!SDL_Vulkan_GetInstanceExtensions(NULL, &extensionCount, extensions))
     {
         SDL_free((void*)extensions);
         SDL_LogError(SDL_LOG_CATEGORY_APPLICATION,