Fix tx_signatures ordering for splices

[t-bast]

We were incorrectly splitting the shared input amount based on each peer's balance, but for the signing order the peer that adds the shared input takes credit for the whole amount of that input.

[codecov-commenter]

Codecov Report

Merging #2768 (9faa3ae) into master (9ca9227) will increase coverage by 0.01%.
Report is 1 commits behind head on master.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master    #2768      +/-   ##
==========================================
+ Coverage   85.84%   85.86%   +0.01%     
==========================================
  Files         216      216              
  Lines       18102    18095       -7     
  Branches      762      746      -16     
==========================================
- Hits        15540    15537       -3     
+ Misses       2562     2558       -4     

Files	Coverage Δ
...re/src/main/scala/fr/acinq/eclair/NodeParams.scala	93.39% <100.00%> (-0.03%)	⬇️
...inq/eclair/channel/fund/InteractiveTxBuilder.scala	91.54% <100.00%> (+0.04%)	⬆️
...cinq/eclair/remote/EclairInternalsSerializer.scala	97.82% <100.00%> (ø)
...scala/fr/acinq/eclair/router/BalanceEstimate.scala	100.00% <100.00%> (+1.06%)	⬆️
.../src/main/scala/fr/acinq/eclair/router/Graph.scala	97.25% <100.00%> (-0.09%)	⬇️
...cala/fr/acinq/eclair/router/RouteCalculation.scala	94.56% <100.00%> (ø)
...src/main/scala/fr/acinq/eclair/router/Router.scala	94.78% <100.00%> (ø)
...main/scala/fr/acinq/eclair/router/Validation.scala	95.00% <100.00%> (ø)

... and 5 files with indirect coverage changes