- add flag +BILLBOARDFACECAMERA - renders actor billboard to always face the camera

tested with this code snippet:
```
class ZombieManFaceCamera : Zombieman replaces Zombieman
{
	default
	{
		+BILLBOARDFACECAMERA;
	}
}
```

src/scripting/thingdef_data.cpp

@@ -378,6 +378,7 @@ static FFlagDef ActorFlagDefs[]=
 	DEFINE_FLAG(RF, NOSPRITESHADOW, AActor, renderflags),
 	DEFINE_FLAG(RF2, INVISIBLEINMIRRORS, AActor, renderflags2),
 	DEFINE_FLAG(RF2, ONLYVISIBLEINMIRRORS, AActor, renderflags2),
+	DEFINE_FLAG(RF2, BILLBOARDFACECAMERA, AActor, renderflags2),
 
 	// Bounce flags
 	DEFINE_FLAG2(BOUNCE_Walls, BOUNCEONWALLS, AActor, BounceFlags),