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.

Use some odd number that will expose problems with remaining samples
after optimized vector operations.

Instead of just adding a rectangular random value before shifting back
to the normal scale, use the existing PcmDither library.

Prepare for adding state.