New Engine Feature: Tweak op138

[4Luke4]

Description:
param2 in {0, 8, 11, 12, 13} and (param1 & 1) ~= 0 -> perform a real attack and call the Lua function in resource before the attack roll / damage is finalized.

Callback signature:

FUNC(op138: CGameEffect, sprite: CGameSprite, baseAttackRoll: number, baseDamageRoll: number) -> newBaseAttackRoll: number, newBaseDamageRoll: number[, boolean]

Notes:

• The callback name must be 8 characters or less, and be ALL UPPERCASE.
• The returned base attack roll is clamped to the engine's natural d20 range [1, 20].
• The returned base damage roll is clamped to >= 0.
• The returned base damage roll replaces only the engine's raw Roll:X basis;
  • later damage modifiers still apply normally unless the optional third return forces final damage to 0.
• If the optional third return is true and the returned base damage roll clamps to 0, final damage is forced to 0 too, ignoring later damage modifiers.

Addendum:
See also: Bubb13/EEex#114.