ao_lavc: Fix advancing of audio pts.
ao_lavc: Add a missing newline for the log.
ao_lavc: Fix design of audio pts handling.

There was confusion about what should go into audio pts calculation and
what not (mainly due to the audio push thread). This has been fixed by
using the playing - not written - audio pts (which properly takes into
account the ao's buffer), and incrementing the samples count only by the
amount of samples actually taken from the buffer (unfortunately this
now forces us to keep the lock too long for my taste).