Comment 9 for bug 1828080

@Seth, @Adrian, I have pushed the above patch with some minor wording changes and a couple of minor variations. It should now work as before in pan-mode, and should not do anything with horizontal wheel events in zoom-mode.

If we thing of something useful to do with horizontal wheel events in zoom-mode, it's easy to add: just add an else block after wx_view_controls.cpp line 221.