lwindex: Pretty hackish workaround for the ASF demuxer.

The Simple Index Object does not always describe all keyframes in
corresponding video stream.
This workaround expects the ASF demuxer imports the indexes from the Simple
Index Object by av_seek_frame() and we replace timestamps stored in the
indexes with actual PTSs since the Simple Index Object has no PTS offset
derived from missing indexes of early keyframes.
In addition, construct the indexes for audio stream from actual timestamps
and positions if present.
lwindex: Make up the indexes without Simple Index Object.

Apparently, this works reliably.
lwindex: Write stream durations only for video or audio streams.
vslsmas: Support VFR->CFR conversion.
VapourSynth/README: Add missing arguments: variable, format.