Backends: SDL/Viewports: Fix crash when SDL backend is used with DirectX graphics API and system does not support Vulkan.

We must pass this flag only when we intend to render using vulkan backend.

examples/imgui_impl_sdl.cpp

@@ -74,6 +74,7 @@ static bool         g_MousePressed[3] = { false, false, false };
 static SDL_Cursor*  g_MouseCursors[ImGuiMouseCursor_COUNT] = {};
 static char*        g_ClipboardTextData = NULL;
 static bool         g_MouseCanUseGlobalState = true;
+static bool         g_UseVulkan = false;
 
 // Forward Declarations
 static void ImGui_ImplSDL2_InitPlatformInterface(SDL_Window* window, void* sdl_gl_context);
@@ -240,6 +241,7 @@ bool ImGui_ImplSDL2_InitForVulkan(SDL_Window* window)
 #if !SDL_HAS_VULKAN
     IM_ASSERT(0 && "Unsupported");
 #endif
+    g_UseVulkan = true;
     return ImGui_ImplSDL2_Init(window, NULL);
 }
 
@@ -467,7 +469,7 @@ static void ImGui_ImplSDL2_CreateWindow(ImGuiViewport* viewport)
     }
 
     Uint32 sdl_flags = 0;
-    sdl_flags |= use_opengl ? SDL_WINDOW_OPENGL : SDL_WINDOW_VULKAN;
+    sdl_flags |= use_opengl ? SDL_WINDOW_OPENGL : (g_UseVulkan ? SDL_WINDOW_VULKAN : 0);
     sdl_flags |= SDL_GetWindowFlags(g_Window) & SDL_WINDOW_ALLOW_HIGHDPI;
     sdl_flags |= SDL_WINDOW_HIDDEN;
     sdl_flags |= (viewport->Flags & ImGuiViewportFlags_NoDecoration) ? SDL_WINDOW_BORDERLESS : 0;