Let the constructor and destructor do this.  This means that all users
have to be converted to allocate PcmConvert dynamically.

The two were never used at the same time, and merging them saves one allocation.

This transfers ownership of the buffer instead of adding another reference.

Fix another memory leak.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/514

A newly allocated buffer doesn't need this call; it only adds overhead
for copying the data.

av_frame_get_buffer() leaks memory if buffers were already allocated.

Fixes one of the memory leaks of https://github.com/MusicPlayerDaemon/MPD/issues/514

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.

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!

Fixes valgrind warning.

Fixes another part of #208, the one which caused the bogus exception.

Wired to Filter::Flush().

Closes #153

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

Thsi filter doesn't make sense as manually configured plugin, because
nobody ever calls volume_filter_set() on it, making it a no-op.

These are not in filter_plugins[] and have dedicated constructors
anyway.

Circumvent filter_new() with a fake ConfigBlock.