The SDL mouse wheel event seems inconsistent between horizontal and
vertical scrolling.
> Movements to the left generate negative x values and to the right
> generate positive x values. Movements down (scroll backward) generate
> negative y values and up (scroll forward) generate positive y values.
<https://wiki.libsdl.org/SDL_MouseWheelEvent#Remarks>
Reverse the horizontal.
Fixes <https://github.com/Genymobile/scrcpy/issues/49>.
f9a63ec272