fixed up handling of WM_MOUSEWHEEL, low order of wParam is not the mouse

wheel number, but indicates which virtual keys are pressed during the
scroll wheel motion

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@1542 7f0cb862-5218-0410-a997-914c9d46530a