If no conversion is necessary, return the child Filter as-is.  This
allows removing all nullptr checks from AutoConvertFilter.

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>

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.

For easier state management inside filter plugins.

The old struct config_param remains only for top-level string options.

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

Replaces Reset() and eliminates the AudioFormat parameters from the
Convert() method.

This prevented using the "volume_normalization" feature with some
codecs (e.g. mp3), because the normalization code requires 16 bit
samples.  If the codec happens to deliver formats other than S16, the
AutoConvert filter succeeds to initialize the conversion filter, but
the returned input audio format was wrong.

Replaces GLib's GError.