MenuStatusCallback when menu was closed did not return location of pointer relative...

MenuStatusCallback when menu was closed did not return location of pointer relative to child window's top-left, it only worked for top level windows.
To make this work, fghPlatformGetCursorPos now can return cursor pos relative to top-left of a specified window's client area (this is untested on X11)

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@1550 7f0cb862-5218-0410-a997-914c9d46530a