Workaround for driver bug observed on the Raspberry Pi, see code
comment for details.

Fixes nullptr dereference in std::string::operator=().  Also fixes a
memory leak by using config_get_block_string() instead of
config_dup_block_string().

.. and not in_frame_size, because this relates to the frame size being
sent to ALSA.  pcm_export_source_size() will then turn it back into
the in_frame_size scale.

Move code from the ALSA output plugin.

The hard-coded "3 bytes" was wrong because it ignored the number of
channels.

DSD-over-USB should not be a MPD core format, because it is not a
"natural" format; it is just a temnporary over-the-wire format.  This
format has been implemented in pcm_export, and does not need to be
supported by pcm_convert.

For 32 bit DSD-over-USB support.

Prepare for removing SAMPLE_FORMAT_DSD_OVER_USB.

Give the caller more control, prepare for DSD-over-USB improvements.

Make it in_frame_size and out_frame_size, to account for packing.

For simplicity, the MPD core should not have to deal with packing.  It
is rarely used, and those plugins that need it should use the
pcm_export library instead.

Converts padded 24 bit samples to packed 24 bit samples.  Will replace
the packed S24 sample format, which is not used internally.

Let the caller configure the audio_format object.

Merge three functions into one and call get_bitformat() only once.

Eliminate support for reverse endian samples from the MPD core.  This
moves a lot of complexity to the plugins that really need it (only
ALSA and CDIO currently).

Don't use audio_format.reverse_endian.

This format is unused since the DSDIFF decoder plugin now reverses the
bit order.

Basic support for Direct Stream Digital.  No conversion yet, and no
decoder/output plugin support.

Support for conversion from float to 16, 24 and 32 bit integer
samples.

Pass audio_output objects around instead of void pointers.  This will
give some more control to the plugin, and prepares for non-blocking
audio outputs.

Consistent naming.

Move the "extern" declarations from output_list.c, for more type
safety.