This has previously been unimplemented, and this diff adds support for all SLP MINT txs.
There are two different variants:
- V1, NFT1 CHILD, NFT1 GROUP: Use mint baton (by assigning an output as the new mint baton) and have exactly 1 mint amount output. See https://github.com/badger-cash/slp-specifications/blob/master/slp-token-type-1.md#mint---extended-minting-transaction for the spec.
- V2 (Mint Vault): Don't use a mint baton, instead require one of the inputs to have a P2SH script with the mint_vault_scripthash as scripthash, and can have 1 to 19 mint amount outputs. See https://github.com/badger-cash/slp-specifications/blob/master/slp-token-type-2.md#mint---extended-minting-transaction for the spec.