Fix left/right scroll on OSX/trackpad

Speculative change - this fixes left/right scrolling for me.  However
it is entirely possible that it breaks left/right scrolling for other
trackpads that issue wheel events - none of my other devices issue
left/right wheel events (and perhaps such devices are few and far
between).  Submitted to see whether anyone else is impacted.
Merge pull request #1542 from PaulL1/tune_mac_scrolling

Fix left/right scroll on OSX/trackpad