[fix] GestureDetector: fix multiswipe direction detection code (#4640)
[fix] GestureDetector: fix multiswipe direction detection code (#4640)

Unfortunately a mistake snuck into the final steps of refactoring <https://github.com/koreader/koreader/pull/4607>.

Thanks to @poire-z for pointing it out [here](https://github.com/koreader/koreader/pull/4607#issuecomment-464856789).
Hzj-jie/koreader,Frenzie/koreader,poire-z/koreader,koreader/koreader,Markismus/koreader,NiLuJe/koreader,koreader/koreader,mwoz123/koreader,pazos/koreader,Frenzie/koreader,NiLuJe/koreader,houqp/koreader,poire-z/koreader,mihailim/koreader