SDLWindow: fix SetVSyncMode return

[ACrazyTown]

I believe the return value here (specifically the res == mode part) is wrong, and might incorrectly report a success. According to the SDL2 documentation, the only valid return values for SDL_GL_SetSwapInterval are 0 or -1:

(int) Returns 0 on success or -1 if setting the swap interval is not supported; call SDL_GetError() for more information.
https://wiki.libsdl.org/SDL2/SDL_GL_SetSwapInterval