Don't ignore trailing empty line

Trailing \N is effectively ignored, but
\N\N should still yield an empty line.
Fix \t corner cases

The end time is reset to line duration if and only if it is zero.
Negative accelerations are allowed (and can cause overflow later).
Fix \fade corner cases

Times in \fade(,,,-1,fadein,fadeout,-1)
are interpreted as in \fad(fadein,fadeout).

Make sure we check the times in the same
order as VSFilter in case they are not sorted.