Added DONTFLIP flag.

- By default, when viewing a flat sprite from behind, the image is flipped around on the X axis. This may not always be desired, so this flag disables it.

src/thingdef/thingdef_data.cpp

@@ -269,6 +269,7 @@ static FFlagDef ActorFlagDefs[]=
 			// [fgsfds] Flat sprites
 	DEFINE_FLAG(RF, FLATSPRITE, AActor, renderflags),
 	DEFINE_FLAG(RF, WALLSPRITE, AActor, renderflags),
+	DEFINE_FLAG(RF, DONTFLIP, AActor, renderflags),
 
 	// Bounce flags
 	DEFINE_FLAG2(BOUNCE_Walls, BOUNCEONWALLS, AActor, BounceFlags),