Improve buffer sizing in `riegeli::tensorflow::FileReader`:
Improve buffer sizing in `riegeli::tensorflow::FileReader`:

- In `PushSlow()`, compute the buffer length based on `pos()` rather than
  `limit_pos()`, because the buffer of that length will also contain available
  data.

- In `ReadSlow()` and `CopySlow()`, subtract `available()` from the recommended
  buffer size, because the buffer of that length will not contain available
  data.

Both cases bring the code closer to `BufferedReader`.

Cosmetics: make variable ordering more consistent with `BufferedReader`, and
introduce local `available_length` variables like in `BufferedReader` (even
though here it is not needed for keeping the value valid across buffer pointer
changes).
PiperOrigin-RevId: 460181865

google/riegeli,google/riegeli,google/riegeli,google/riegeli