`AVCodecParameters` contains values from the codec detected by
avformat_find_stream_info(), but after avcodec_open2(), a different
codec might be selected with a different `AVSampleFormat`.  This leads
to misinterpretation of data returned from FFmpeg, leading to random
noise or silence.

This was observed with FFmpeg 4.0.2 and a TS container file containing
MP2.  A mp3-float codec was detected returning `AV_SAMPLE_FMT_FLTP`,
but finally the `mpegaudiodec_fixed.c` was used, returning
`AV_SAMPLE_FMT_S16`.

By using the audio format from `AVCodecContext`, we ensure that MPD
and FFmpeg always agree on the actual audio format in the buffer.

This removes the FFmpeg bug workaround from commit e1b032cb which I
assume is obsolete after 7 years.

Fixes #380

Drop some compatibility code.

Closes #360

Use `const_cast` to be able to pass it to sf_open_virtual() which has
a wrong parameter declaration.

So long, autotools!  This is my last MPD related project to migrate
away from it.  It has its strengths, but also very obvious weaknesses
and weirdnesses.  Today, many of its quirks are not needed anymore,
and are cumbersome and slow.  Now welcome our new Meson overlords!

Broken since commit 08f77c2b

This is the version in Debian Jessie (oldstable).

This is kind of a revert of commit
b2e3c075, which is not any longer
necessary since commit 0dd4b52b
removed the last call to `av_open_input_stream()`.

Remove another dependency on the config/Global library.