[SDL] SDL_Flip doesn't wait vsync under X11

[Ryan C. Gordon]

> It doesn't work with SDL under X11, wich mean that every SDL applications runs 
> with tearing artefacts under X11. IMHO it is a CRITIAL bug. I know it's due to 
> X11, but a solution must be found NOW !

Right now, huh?

The docs say your surface must have the HWSURFACE flag...even if you 
request it on X11, the SDL_Surface returned by SDL_SetVideoMode() won't 
have it. X11 doesn't expose that functionality to clients.

If you have a good way to make it happen, we'll implement it. But I 
think it would take an X11 extension to do so.

(It's worth noting that glX _can_ be set to do real hardware page 
flipping, if you can use OpenGL...we expose this with 
SDL_GL_SWAP_CONTROL in SDL_GL_SetAttribute(), and there are environment 
variables in the Nvidia drivers to force it on GL contexts, too.)

--ryan.