Treat zero-area events as explicitly positioned
Treat zero-area events as explicitly positioned

VSFilter does collision by calculating the intersection of two events
rectangles and treating events as colliding iff their intersection
has a non-zero area.
As a consequence zero-area events cannot collide with anything.
https://github.com/Cyberbeing/xy-VSFilter/blob/xy_sub_filter_rc4@%7B2020-05-17%7D/src/subtitles/RTS.cpp#L1627

libass does not do collision by intersection area, but because we
ignore edge-edge intersections results are identical to VSFilter
for non-zero-area events.

To reconcile the results for zero-area events, treat them as explicitly
positioned as they effectively are in VSFilter. This allows us to keep
our (probably) faster collision code and saves us from unnecessary
collsion checks for zero-area events.

Fixes https://github.com/libass/libass/issues/444

libass/libass,MrSmile/libass,Akemi/libass,MrSmile/libass,ShiftMediaProject/libass,xbmc/libass,xbmc/libass,astiob/libass,Akemi/libass,astiob/libass,ShiftMediaProject/libass,Akemi/libass,libass/libass