dec_video: remove unused declaration of a former global variable
vd_lavc: add more ifdeffery and ffmpeg cargo culting for correctness

We mixed the "old" AVFrame management functions (avcodec_alloc_frame,
avcodec_free_frame) with reference counting. This doesn't work
correctly; you must use av_frame_alloc and av_frame_free. Of course
ffmpeg doesn't warn us about the bad usage, but will just mess up
things silently. (Thanks a lot...)

While the alloc function seems to be 100% compatible, the free function
will do bad things, such as freeing memory that might still be
referenced by another frame. I didn't experience any actual bugs, but
maybe that was pure luck.
ao_pulse: fix channel layouts

The code was selecting PA_CHANNEL_POSITION_MONO for MP_SPEAKER_ID_FC,
which is correct only with the "mono" channel layout, but not anything
else. Remove the mono entry, and handle mono separately.

See github issue #326.
sd_lavc: display DVD subs with unknown duration

DVD subs (rarely) have subtitle events without end timestamp. The
duration is unknown, and they should be displayed until they're
replaced by the next event.

FFmpeg fails hard to make us aware whether duration is unknown or
actually 0, so we can't distinguish between these two cases. It fails
at this twice: AVPacket.duration is set to 0 if duration is unknown,
and AVSubtitle.end_display_time has the same issue.

Add a hack that considers all bitmap subtitles with duration==0 as
events with uknown length. I'd rather accidentally display a hidden
subtitle (if they exist at all), instead of not displaying random
subtitles at all.

See github issue #325.