Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

This fixes a valgrind warning because `buffer_before_play`
initialization needs to know the audio format from the decoder.

This emits the event even if PlayerControl::Play() is used to replay
the current song, which emits the missing "player" idle event.

Closes #381

Keep MPD's footprint small until playback is requested.

Previously, there was the setting `buffered_before_play` which
specified a percentage of the audio buffer, defaulting to `10%`.  That
was working well enough for quite some time, until high-quality audio
formats became common.

At 44.1 kHz, 16 bit stereo, MPD collected 2.3 seconds worth of data in
the buffer before starting playback.  With the same default settings
and 192 kHz, 24 bit stereo, that was only 0.27 seconds.

Making this depend on the byte size only leads to high latency at low
quality, and too little data at high quality.  The natural choice
would be to use a duration instead of a byte size, which should give
the same good experience with all audio formats.

Since the `buffered_before_play` configuration setting was not
understood well by users and caused more harm than good, this commit
deprecates it.  It has now no effect.

Simplify the formula, and I guess this makes the formula more
reliable.  Imagine somebody configured `buffered_before_play` larger
than 25%; then the decoder would be woken up all the time.  This
doesn't seem logical.  On the other hand, it's easy to understand that
the decoder should be woken up below 75% buffer fill.

Calculate the value only once.

Shouldn't ever happen, but who knows...

Since commit e81b0896, this is the
same code as in ReplacePipe().

This requires re-adding the reference to struct DecoderControl, which
was removed recently by commit
9f14e7a9

Make all uses of MusicChunk safe.

We can implement this now that we have a MusicBuffer reference.

This tiny amount of overhead allows omitting the MusicBuffer in
Clear().

This mostly affects the Pulse output plugin which needs to "cork" the
stream (closes #278).

When switching to another song manually, the player checks if the
decoder is already decoding that song; if so, it will attempt to reuse
it by seeking it to the new position.  That however fails if the
decoder is not seekable (e.g. a radio stream) which leaves the user
unable to switch to that song with the bogus error message "Not
seekable".