some cleanup

now have function fghPlatformGetMousePos to get current mouse position
in screen coordinates
Using this in fg_menu as all that code should be platform independent

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@1376 7f0cb862-5218-0410-a997-914c9d46530a