macosx: Refine scroll wheel event forwarding
macosx: Refine scroll wheel event forwarding

Starting with Sierra, the platform emits more fine granular
scrolling events than before. Now, the deltas from each event
are accumulated and forwarded after they reach a certain threshold.

close #17349

xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc