audio: prefer libavcodec over libmpg123

libavcodec/libavformat now handles gapless audio better. In theory, this
could be implemented with ad_mpg123 too, but since libavformat strips
metadata from mp3 files and passes pure mp3 packets to the decoders
only, this can't work by itself. Instead, the player must pass this
metadata separately. libav* do this relatively transparently over packet
"side data" (attached to AVPacket).

It might also be possible to let libmpg123 handles all this by
implementing it as demuxer that outputs PCM, but that would have other
problems, and I think it's better to make libavformat work correctly.

libmpg123 can still be used with '--ad=mpg123:mp3'.

Also see issue #1101.
player: allow passing number of loops to --loop-file

E.g. --loop-file=2 will play the file 3 times (one time normally, and 2
repeats).

Minor syntax issue: "--loop-file 5" won't work, you have to use
"--loop-file=5". This is because "--loop-file" still has to work for
compatibility, so the "old" syntax with a space between option name and
value can't work.
vf_vapoursynth: make it possible to get filter output incrementally

Until now, we always required the playback core to decode a new frame to
get more output from the filter. That seems to be completely
unnecessary, because filtered results may arrive before that.

Add a filter_out callback, and restructure the code such that it can
return any filtered frames, or block if it hasn't read at least one
frame.

In the worst case, it still can happen that bursts of input requests and
output requests happen. (This commit tries to reduce burst-like
behavior, but it's not entirely possible due to the indeterministic
nature of VS threading.)

This is a similar change as with 95bb0bb6.