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.
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).
