vo_lavc: fix -ovoffset.

Previously, using it led to no single frame being output, ever.
ao_lavc: remove audio offset hack to ease supporting planar audio.

Now to shift audio pts when outputting to e.g. avi, you need an explicit
facility to insert/remove initial samples, to avoid initial regions of
the video to be sped up/slowed down.

One such facility is the delay filter in libavfilter.