Adding support for GLX/WGL_ARB_multisample querying

Hi all,

i would like to add support for querying the GLX_ARB_multisample & WGL_ARB_multisample extensions to make the supported multisample params for each visual/pixel format available to the user.

While exploring the 1.3 codebase i’ve found the SDL_DisplayMode struct that seems a good place to add these params.

Now some questions to the devs here:

What do you think about adding the params to SDL_DisplayMode? OK or not? If not: Where else?
I thought of setting the multisample params in SDL_x11modes.c::X11_GetDisplayModes and SDL_win32modes.c::WIN_GetDisplayMode. Good idea or not? Anything i need to take care about? Any other places that need changes?