Vittorio Giovara      2011-02-01 02:21:50 PST

with the attached patch, the opengles context will always use the maximum
screensize available; this is particularly useful for supporting retina display
on latest iphone.

please note: Apple documentation warns that using the "upscaled" gl context
actually uses more memory and bandwitdh, so it might be worth to let the user
decide whether to disable it or not, either with a flag or a sdl function...