When a window is iconified, only an UnmapNotify is sent, not a

When a window is iconified, only an UnmapNotify is sent, not a
VisibilityNotify, so we have to handle the window status callback in the
former case, too. This fixes bug #763442 (Call the visibility callback when
minimizing a window).

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@649 7f0cb862-5218-0410-a997-914c9d46530a