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.

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.

Replaces GLib's GError.

Make the audio_format argument non-const.  Allow the open() method to
modify it, to indicate that it wants a different input audio format
than the one specified.  Check that condition in chain_filter_open(),
and fail.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

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