- remove the SG_GLxxxx_H #defines, since OSG provides its own versions

 - remove the SG_GLxxxx_H #defines, since OSG provides its own versions
 - this exposed a bizarre issue on Mac where dragging in <AGL/agl.h> in
   extensions.hxx was pulling in all of Carbon to the global namespace
   - very scary. As a result, I now need to explicitly include CoreFoundation
   in fg_init.cxx.
 - change SG_USING_STD(x) to using std::x

Issues:

 - the logic for X11 and Win32 in RenderTexture and extensions is tortured,
   please see if you agree I got all the ifdefs correct.