Review sig check in masp vp

[grarco]

This is a possible(?) breaking improvement of the signature check in the MASP VP.

Could the signature check in the masp vp be done only if the balances decrease more than implied by the Transaction?
As in, if we expect a balance to go down by 10 but we see a decrease of 15 we request a signature since this means that something else happened in parallel to the masp tx that we cannot verify in here. Otherwise, if the decrease matches (or is less than) the exepected value of Transaction we don't require/verify the signature. Could this also apply to validate_transparent_input?

This (if correct) could lead to less signatures needed on the transaction and less verifications.

Originally posted by @grarco in #3186 (comment)