`gadget:ShieldPreDamaged` should receive hp and damage and return damage

[sprunk]

Shield HP and incoming projectile damage sound pretty important to know when handling a hit, and it's even impossible to get the damage correctly in some cases.

Additionally the damage would be good to modify, like you can with UnitPreDamaged.

[sprunk]

Tips:

• add two args to the function and pass them along so they end up in LuaHandle::ShieldPreDamaged.
• move the event being called (see picture below) so you can pass shieldDamage and curPower.
• add two lua_pushnumbers inside LuaHandle::ShieldPreDamaged, after existing args.
• remove the const from damage and pass it as a reference so it can be modified. Look at UnitPreDamaged for how to use the value returned from Lua to modify the C++ value.