When making the window centered it should use windowed mode size since it doesn't affect fullscreen windows

Fixes bug https://github.com/libsdl-org/SDL/issues/4750

src/video/SDL_video.c

@@ -2045,10 +2045,10 @@ SDL_SetWindowPosition(SDL_Window * window, int x, int y)
 
         SDL_GetDisplayBounds(displayIndex, &bounds);
         if (SDL_WINDOWPOS_ISCENTERED(x)) {
-            x = bounds.x + (bounds.w - window->w) / 2;
+            x = bounds.x + (bounds.w - window->windowed.w) / 2;
         }
         if (SDL_WINDOWPOS_ISCENTERED(y)) {
-            y = bounds.y + (bounds.h - window->h) / 2;
+            y = bounds.y + (bounds.h - window->windowed.h) / 2;
         }
     }