Another step towards supporting multiple readers.

Prepare to reuse it in another class.

Just like std::condition_variable, which however has no way to specify
the std::mutex directly.

Previously, a seek to the end of the file would cause an assertion
failure in SparseMap::Check() because the given offset was invalid.

Closes #453

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.

Instead of copying the offset from our `input`, copy the requested
offset to our `offset` attribute.

By the time Seek() finishes, our input's offset may have advanced
already, having read some more data, thus giving us a bogus offset.

This fixes spurious decoder failues (closes #320).