Code review comment for lp://qastaging/~azzar1/nux/lp-1228093

> Removing NUX_EVENT_MOUSEWHEEL will break the API.

You don't have to remove it.
That bit will be shared by both the movements so you'll have:

_mouse_state |= NUX_EVENT_MOUSEWHEEL;
_mouse_state |= NUX_EVENT_MOUSEWHEEL_HORIZONTAL;

and
_mouse_state |= NUX_EVENT_MOUSEWHEEL;
_mouse_state |= NUX_EVENT_MOUSEWHEEL_VERTICAL;