Fixed a few compiler warnings and compilation on OS X

@alexeevdv, look at [https://github.com/alexeevdv/falltergeist/blob/bbc344d8aec18b364505b6eed1b9fdfd77f6fab7/src/States/CursorDropdownState.cpp#L175-L179](these lines). They don't make sense unless `_currentSurface` is signed.