SDL: Respect OS setting for wheel scroll direction on SDL 2.0.4+
The setting was already respected on SDL < 2.0.4 (such as SDL1). If the OS inverts the scrolling direction, this is reflected on the sign of the y value of the SDL event. Since version 2.0.4 the SDL event also had a flag to indicate if the direction is flipped and we were using it to change back the y sign. That means the OS scrolling direction setting was not respected. With this commit we now have a consistent behaviour with all SDL versions (the OS scroll direction setting is respected).
If our wheel events are used for something other than scrolling however, we might want to get the actually wheel move direction (i.e. use a Common::EVENT_WHEELDOWN when moving the wheel down even if the OS scroll setting is to flip it). In such a case we might want to revert this change and instead add an additional flag to the event so that we know the scroll direction should be flipped and use it in places where the event is used for scrolling.