Add a hint for D3D9Ex to avoid having to choose at compile-time

include/SDL_hints.h

@@ -1238,6 +1238,26 @@ extern "C" {
  */
 #define SDL_HINT_WINDOWS_FORCE_SEMAPHORE_KERNEL "SDL_WINDOWS_FORCE_SEMAPHORE_KERNEL"
 
+/**
+ * \brief Use the D3D9Ex API introduced in Windows Vista, instead of normal D3D9.
+ *        Direct3D 9Ex contains changes to state management that can eliminate device
+ *        loss errors during scenarios like Alt+Tab or UAC prompts. D3D9Ex may require
+ *        some changes to your application to cope with the new behavior, so this
+ *        is disabled by default.
+ *
+ *  This hint must be set before initializing the video subsystem.
+ *
+ *  For more information on Direct3D 9Ex, see:
+ *    - https://docs.microsoft.com/en-us/windows/win32/direct3darticles/graphics-apis-in-windows-vista#direct3d-9ex
+ *    - https://docs.microsoft.com/en-us/windows/win32/direct3darticles/direct3d-9ex-improvements
+ *
+ *  This variable can be set to the following values:
+ *    "0"       - Use the original Direct3D 9 API (default)
+ *    "1"       - Use the Direct3D 9Ex API on Vista and later (and fall back if D3D9Ex is unavailable)
+ *
+ */
+#define SDL_HINT_WINDOWS_USE_D3D9EX "SDL_WINDOWS_USE_D3D9EX"
+
 /**
  * \brief Tell SDL which Dispmanx layer to use on a Raspberry PI
  *