now that mouse capture was properly implemented, menus could be opened

now that mouse capture was properly implemented, menus could be opened
outside the window when the window had capture because another mouse
button was depressed. fixed
Also implemented WM_CANCELMODE, which could be important as we should
cancel capture when it comes in

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@1526 7f0cb862-5218-0410-a997-914c9d46530a