Minor corrections to the UNIX_X11 mouse-wheel support. In part this

Minor corrections to the UNIX_X11 mouse-wheel support.  In part this
reflects that {button} is already shifted to the GLUT numbering, and
hence should be counted as 3&4, 5&6, etc.

The other change was simply to clarify the computation of {direction}.
The original code did some clever arithmetic.  The new code is more
straightforward, even if it requires more lines to express.

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@306 7f0cb862-5218-0410-a997-914c9d46530a