Visibility functions better implemented now. All goes through

Visibility functions better implemented now. All goes through
WM_SHOWWINDOW except for a newly created window (WM_SHOWWINDOW is too
early for callback then). I now have a state variable that indicates
whether visibility has been notified yet for a new window. This
notification gets done just before the window is first drawn. This
removes the need for fgPlatformMainLoopPreliminaryWork on windows, and
it also works for windows created when the mainloop is already running

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@1541 7f0cb862-5218-0410-a997-914c9d46530a