WOF_ADD for A_OverlayAlpha

[Jekyll Grim Payne]

It's very easy to implement fade-in/fade-out in ZScript, but I feel like adding it to the native function would be good for consistency, since other A_Overlay* manipulation functions use that parameter in some way.

My idea is to simply expand it to A_OverlayAlpha(int layer, double alph, int flags = 0) where the flags argument recognizes WOF_ADD and performs an incremental change instead of an absolute one.

I'd add PSPF_REMOVE as a flag option as well, to make it move to the "Null" state sequence if alpha <= 0 (similarly to FTF_REMOVE in A_FadeOut). Or it could be just another boolean argument.