Reverse horizontal scrolling behavior

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>.

app/src/convert.c

@@ -172,7 +172,10 @@ SDL_bool mouse_wheel_from_sdl_to_android(const SDL_MouseWheelEvent *from,
     to->scroll_event.position = position;
 
     int mul = from->direction == SDL_MOUSEWHEEL_NORMAL ? 1 : -1;
-    to->scroll_event.hscroll = mul * from->x;
+    // SDL behavior seems inconsistent between horizontal and vertical scrolling
+    // so reverse the horizontal
+    // <https://wiki.libsdl.org/SDL_MouseWheelEvent#Remarks>
+    to->scroll_event.hscroll = -mul * from->x;
     to->scroll_event.vscroll = mul * from->y;
 
     return SDL_TRUE;