Fix crash when Fl_Window::flush() is called before show() (STR #3028).

The fix is twofold:
 (1) if Fl_Window::flush() is called, this is ignored silently
 (2) if Fl_Window::make_current() is called directly, but the window
     is currently not shown(), then the program will be terminated
     with an error message.
The latter is necessary because returning from make_current() would
leave the program in a state where it would expect to have a valid
drawing context.


git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10095 ea41ed52-d2ee-0310-a9c1-e6b18d33e121