restore old fadestep behavior if SPF_NEGATIVE_FADESTEP is not passed in

Author: RicardoLuis0

src/playsim/p_effect.cpp

@@ -374,7 +374,7 @@ void P_SpawnParticle(FLevelLocals *Level, const DVector3 &pos, const DVector3 &v
 		particle->Acc = FVector3(accel);
 		particle->color = ParticleColor(color);
 		particle->alpha = float(startalpha);
-		if (fadestep <= -1.0) particle->fadestep = FADEFROMTTL(lifetime);
+		if ((fadestep < 0 && !(flags & SPF_NEGATIVE_FADESTEP)) || fadestep <= -1.0) particle->fadestep = FADEFROMTTL(lifetime);
 		else particle->fadestep = float(fadestep);
 		particle->ttl = lifetime;
 		particle->size = size;

src/playsim/p_effect.h

@@ -65,6 +65,7 @@ enum EParticleFlags
 	SPF_REPLACE					= 1 << 7,
 	SPF_NO_XY_BILLBOARD			= 1 << 8,
 	SPF_LOCAL_ANIM				= 1 << 9,
+	SPF_NEGATIVE_FADESTEP		= 1 << 10,
 };
 
 class DVisualThinker;

wadsrc/static/zscript/constants.zs

@@ -714,6 +714,7 @@ enum EParticleFlags
 	SPF_REPLACE					= 1 << 7,
 	SPF_NO_XY_BILLBOARD			= 1 << 8,
 	SPF_LOCAL_ANIM				= 1 << 9,
+	SPF_NEGATIVE_FADESTEP		= 1 << 10,
 
 	SPF_RELATIVE				= SPF_RELPOS|SPF_RELVEL|SPF_RELACCEL|SPF_RELANG
 };