Found with cppcoreguidelines-special-member-functions
Signed-off-by: Rosen Penev <rosenp@gmail.com>

The former was deprecated with C++14.

According to the C++11 and C++17 standards, both files are identical.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

This will be used by filters which have internal buffers which need to
be flushed at the end, e.g. the "soxr" resampler.

Not used.  Force implementations to initialize out_audio_format.

For easier state management inside filter plugins.

API simplification.  We can now avoid abusing a "size_t*" as
additional return value.

Replaces GLib's GError.

The filter API allows us to implement software volume as a pluggable
filter, and we will be able to integrate libraries like SoX.

This patch allows the output plugins to import only mixer_list.h,
instead of the full mixer_api.h (which would expose internal
structures).

mixer_control.h should provide the functions needed to manipulate a
mixer, without exposing the internal mixer API (which is provided by
mixer_api.h).