- add flag +BILLBOARDFACECAMERA - renders actor billboard to always f…

…ace the camera

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

src/playsim/actor.h

@@ -481,21 +481,22 @@ enum ActorRenderFlag
 	RF_MASKROTATION		= 0x00200000, // [MC] Only draw the actor when viewed from a certain angle range.
 	RF_ABSMASKANGLE		= 0x00400000, // [MC] The mask rotation does not offset by the actor's angle.
 	RF_ABSMASKPITCH		= 0x00800000, // [MC] The mask rotation does not offset by the actor's pitch.
-	RF_INTERPOLATEANGLES		= 0x01000000, // [MC] Allow interpolation of the actor's angle, pitch and roll.
+	RF_INTERPOLATEANGLES = 0x01000000, // [MC] Allow interpolation of the actor's angle, pitch and roll.
 	RF_MAYBEINVISIBLE	= 0x02000000,
 	RF_DONTINTERPOLATE	= 0x04000000,	// no render interpolation ever!
 
 	RF_SPRITEFLIP		= 0x08000000,	// sprite flipped on x-axis
 	RF_ZDOOMTRANS		= 0x10000000,	// is not normally transparent in Vanilla Doom
 	RF_CASTSPRITESHADOW = 0x20000000,	// actor will cast a sprite shadow
 	RF_NOINTERPOLATEVIEW = 0x40000000,	// don't interpolate the view next frame if this actor is a camera.
-	RF_NOSPRITESHADOW = 0x80000000,		// actor will not cast a sprite shadow
+	RF_NOSPRITESHADOW	= 0x80000000,	// actor will not cast a sprite shadow
 };
 
 enum ActorRenderFlag2
 {
 	RF2_INVISIBLEINMIRRORS		= 0x0001,	// [Nash] won't render in mirrors
 	RF2_ONLYVISIBLEINMIRRORS	= 0x0002,	// [Nash] only renders in mirrors
+	RF2_BILLBOARDFACECAMERA		= 0x0004,	// Sprite billboard face camera (override gl_billboard_faces_camera)
 };
 
 // This translucency value produces the closest match to Heretic's TINTTAB.

src/rendering/hwrenderer/scene/hw_drawlist.cpp

@@ -523,7 +523,7 @@ void HWDrawList::SortSpriteIntoWall(HWDrawInfo *di, SortNode * head,SortNode * s
 		const bool drawWithXYBillboard = ((ss->particle && gl_billboard_particles) || (!(ss->actor && ss->actor->renderflags & RF_FORCEYBILLBOARD)
 			&& (gl_billboard_mode == 1 || (ss->actor && ss->actor->renderflags & RF_FORCEXYBILLBOARD))));
 
-		const bool drawBillboardFacingCamera = gl_billboard_faces_camera;
+		const bool drawBillboardFacingCamera = gl_billboard_faces_camera || (ss->actor && ss->actor->renderflags2 & RF2_BILLBOARDFACECAMERA);
 		// [Nash] has +ROLLSPRITE
 		const bool rotated = (ss->actor != nullptr && ss->actor->renderflags & (RF_ROLLSPRITE | RF_WALLSPRITE | RF_FLATSPRITE));
 

src/rendering/hwrenderer/scene/hw_sprites.cpp

@@ -386,7 +386,7 @@ bool HWSprite::CalculateVertices(HWDrawInfo *di, FVector3 *v, DVector3 *vp)
 		//&& di->mViewActor != nullptr
 		&& (gl_billboard_mode == 1 || (actor && actor->renderflags & RF_FORCEXYBILLBOARD))));
 
-	const bool drawBillboardFacingCamera = gl_billboard_faces_camera;
+	const bool drawBillboardFacingCamera = gl_billboard_faces_camera || !!(actor->renderflags2 & RF2_BILLBOARDFACECAMERA);
 	// [Nash] has +ROLLSPRITE
 	const bool drawRollSpriteActor = (actor != nullptr && actor->renderflags & RF_ROLLSPRITE);
 
@@ -1135,7 +1135,9 @@ void HWSprite::Process(HWDrawInfo *di, AActor* thing, sector_t * sector, area_t
 		// This is a non-translucent sprite (i.e. STYLE_Normal or equivalent)
 		trans = 1.f;
 
-		if (!gl_sprite_blend || modelframe || (thing->renderflags & (RF_FLATSPRITE | RF_WALLSPRITE)) || gl_billboard_faces_camera)
+		if (!gl_sprite_blend || modelframe ||
+			(thing->renderflags & (RF_FLATSPRITE | RF_WALLSPRITE)) || gl_billboard_faces_camera ||
+			thing->renderflags2 & RF2_BILLBOARDFACECAMERA)
 		{
 			RenderStyle.SrcAlpha = STYLEALPHA_One;
 			RenderStyle.DestAlpha = STYLEALPHA_Zero;

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),