Don't use padding in add_piece?

[th7nder]

          The alternative [`write_and_preprocess`](https://github.com/filecoin-project/rust-fil-proofs/blob/266acc39a3ebd6f3d28c6ee335d78e2b7cea06bc/filecoin-proofs/src/api/mod.rs#L502) just calls `add_pieces` and forces it to not add a padding, so the whole responsibility falls on the sealing logic and the pipeline. I'd say I'm fine for now with this padding and losing a bit of space, to reduce the complexity of other logic.

Originally posted by @th7nder in #434 (comment)

[jmg-duarte]

Moving to after P3, currently this works so there's no point in changing right now