Ooops. We don't want people to be able to create a subwindow of an

Ooops.  We don't want people to be able to create a subwindow of an
offscreen window.  (^&  This is no longer permitted.

(Actually, it would be nice if it would work, but I don't think that you
can subwindow a pixmap in UNIX_X11, and assume that subwindowing offscreen
doesn't work on WIN32, either.)

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@456 7f0cb862-5218-0410-a997-914c9d46530a