Adds GLUT_OFFSCREEN as a display mode option. This includes:

Adds GLUT_OFFSCREEN as a display mode option.  This includes:

 * Recognizing the mode when you call glutCreateWindow().
   (Offscreen looks and acts like a top-level window in many
   ways.)

 * During the life of the offscreen "window", various window-management
   functions are modified to respect offscreen status.

   (Excepting reshaping windows, there is not much useful interpretation
    for managment of offscreen areas.  Reshaping is possibly more work
    than it's worth to support, and GLUT specs allow us to ignore the
    request.)

 * Cleanup for "offscreen" windows is a little different than for regular
   windows.

Windows still don't start up quite ideally.  Offscreen windows should
probably get a reshape event, but not a display.

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@451 7f0cb862-5218-0410-a997-914c9d46530a