Fix incorrect invulnerability damage reduction

[lynxplay]

Fixes incorrect spigot handling of the invulnerability damage
reduction applied when an already invulnerable entity is damaged with a
larger damage amount than the initial damage.
Vanilla still damages entities even if invulnerable if the damage to be
applied is larger than the previous damage taken. In that case, vanilla
applies the difference between the previous damage taken and the
proposed damage.

Spigot's damage modifier API takes over the computation of damage
reducing effects, however spigot invokes this handling with the initial
damage before computing the difference to the previous damage amount.
This leads to the reduction values to generally be larger than expected,
as they are computed on the not-yet-reduced value.
Spigot applies these reductions after calling the EntityDamageEvent and
then subtracts the previous damage point, leading to the final damage
amount being smaller than expected.

This patch cannot simply call the EntityDamageEvent with the reduced
damage, as that would lead to EntityDamageEvent#getDamage() returning
the already reduced damage, which breaks its method contract.
Instead, this patch makes use of the DamageModifier API, implementing
the last-damage-reduction as a DamageModifier.

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Download the paperclip jar for this pull request: paper-11599.zip

[Malfrador]

Just so it doesn't get lost:
I've tested this PR and it seems to fix the issue. Damage is behaving correctly again, as it does in Vanila. In my tests, changing damage in the EntityDamageEvent now also works correctly, which fixes #11480 as well.

Further testing with the build attached to the PR would be welcome though, especially if you have a plugin that changes damage in the EntityDamageEvent/EntityDamageByEntityEvent, to make sure this change doesn't cause unexpected behaviour there.