[Poly] Additive sprite rendering has no upper bound

[GameFreakDude]

I noticed that paletted additive transparency calculations in the poly renderer will happily go past full white and start using garbage instead.

Attached is a sample mod with a white orb in a little room.

[dpJudas]

So there are someone using the palette version of softpoly.

[ZZYZX]

That looks like a nice portal effect, noted for later usage

[dpJudas]

Fixed.