Interpret timestamps with negative components more obviously
Interpret timestamps with negative components more obviously

Keep all components signed, allowing for timestamps like 00:01:-05.00
to denote 55 seconds. This matches the behavior of VSFilter.

libass/libass,MrSmile/libass,mwgoldsmith/libass,ShiftMediaProject/libass,xbmc/libass,mwgoldsmith/libass,MrSmile/libass,xbmc/libass,grigorig/libass,mwgoldsmith/libass,ShiftMediaProject/libass,grigorig/libass,stps/libass,stps/libass,stps/libass,astiob/libass,Akemi/libass,Akemi/libass,Akemi/libass,astiob/libass,libass/libass