Our custom scroll handler for horizontal (Shift+Scroll) and page
(Alt+Scroll) scroll didn't properly check the scroll direction and
assume that if it's not down it's up.  This was mostly not a problem
because the other types only were left and right scroll events, which
are a lot less common.

However, it became a lot more problematic with GTK 3.4 that introduced
"smooth scrolling", and thus a new scroll type that can happen for
events in any direction.  We then would scroll up (as we assume "not
down" is up) regardless of the actual direction of the event.

It's still not clear why we'd get smooth scroll events on X11 as no
code I can find asks for it and we generally don't get those, but
sometimes a Scintilla widget starts receiving them, leading to the bug.
On Wayland on the other hand, Scintilla asks for smooth scroll events,
so we need to have a fix for it in any case.