Be explicit that SDL_FLIP_HORIZONTAL_AND_VERTICAL combines horizontal and vertical flipping

Closes https://github.com/libsdl-org/SDL/pull/13821

include/SDL3/SDL_surface.h

@@ -96,10 +96,10 @@ typedef enum SDL_ScaleMode
  */
 typedef enum SDL_FlipMode
 {
-    SDL_FLIP_NONE,                      /**< Do not flip */
-    SDL_FLIP_HORIZONTAL,                /**< flip horizontally */
-    SDL_FLIP_VERTICAL,                  /**< flip vertically */
-    SDL_FLIP_HORIZONTAL_AND_VERTICAL,   /**< flip horizontally and vertically (not a diagonal flip) */
+    SDL_FLIP_NONE,                                                                  /**< Do not flip */
+    SDL_FLIP_HORIZONTAL,                                                            /**< flip horizontally */
+    SDL_FLIP_VERTICAL,                                                              /**< flip vertically */
+    SDL_FLIP_HORIZONTAL_AND_VERTICAL = (SDL_FLIP_HORIZONTAL | SDL_FLIP_VERTICAL)    /**< flip horizontally and vertically (not a diagonal flip) */
 } SDL_FlipMode;
 
 #ifndef SDL_INTERNAL