Use shared input's txOut in shouldSignFirst

[t-bast]

This is clearer that way and yields the same result: we only look at txOut amounts when computing each node's contributions.

See a related lightning-kmp PR: ACINQ/lightning-kmp#724

[remyers]

When I double checked the spec I did not see that the shared input should be considered at all. What is the rational for counting the total shared input towards towards the amount of the initiating node?

Based on the current wording of the spec it seems we should only count the tx_add_input values and ignore the shared input:

The sending node:
  - if it has the lowest total satoshis contributed, as defined by total
    `tx_add_input` values, or both peers have contributed equal amounts
    but it has the lowest `node_id` (sorted lexicographically):
    - MUST transmit their `tx_signatures` first

[t-bast]

The shared input is sent to the other node with tx_add_input, by the splice initiator:

The sending node:
  - If it is the splice initiator:
    - MUST add the current channel input to the splice transaction by
      sending `tx_add_input` with `shared_input_txid` containing the
      `txid` of the latest funding transaction.

Since there's no reason to explicitly exclude this tx_add_input (I don't see why we'd want to exclude it?), we thus need to make its amount count for the splice initiator.

[remyers]

LGTM! thanks for the explanation of why the initiator should also count the sharedInput towards their total.