Added SpriteAngle and SpriteRotation properties.

- Includes four functions, A_SetSprite(Angle/Rotation) and GetSprite(Angle/Rotation).
- SpriteRotation offsets the angle of the sprite, allowing for actors to move backwards or sideways for example.
- SpriteAngle requires +SPRITEANGLE and sets the actor's sprite to the absolute rotation found at that angle. Overrides SpriteRotation once the flag is on.

src/p_mobj.cpp

@@ -383,6 +383,11 @@ void AActor::Serialize(FArchive &arc)
 		<< RipLevelMin
 		<< RipLevelMax;
 	arc << DefThreshold;
+	if (SaveVersion >= 4549)
+	{
+		arc << SpriteAngle;
+		arc << SpriteRotation;
+	}
 
 	{
 		FString tagstr;