Since we switched from autotools to Meson in commit
94592c1406, 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.
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.
