[SDL] Minor bug on Mac OS X Quartz driver

[Andrea Mazzoleni]

In Mac OS X with the Quartz driver the wm_available flag returned
by SDL_GetVideoInfo() is always 0.

I suppose it should be 1. As workarond now I check if the SDL_ListModes
function return -1, meaning that any screen dimension is available, i.e. you
display in a window.

-- 
Andrea Mazzoleni
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