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.

Closes #208.

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

Wohooooo, the method Filter::Reset() has been broken because no
implementation of it has ever been called for a loooong time.
And nobody ever noticed it.  WTF.

For easier state management inside filter plugins.