Fix for STR#3142 where fl_read_image() correctly reads GL data under X11, but ignores them under MSWindows and Mac OS X.
Moreover, fl_read_image() behaves differently with and without OS virtualization for X11 and MSWindows.
The patched function reads whatever is in the rectangle transmitted in arguments, with and without GL data, with and without subwindows,
on ‘true’ OS or on virtualized OS.


git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10436 ea41ed52-d2ee-0310-a9c1-e6b18d33e121