attempt to disable screen savers. Works for Quartz (Mac OS X) and X11.

Need a formal API for this in 1.3, still.

  Fixes Bugzilla #415.

--HG--
branch : SDL-1.2
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/branches/SDL-1.2%402305