VarAction2: Add an ABS opcode

src/newgrf_spritegroup.h

@@ -198,6 +198,7 @@ enum DeterministicSpriteGroupAdjustOperation : uint8 {
 	DSGA_OP_SGT,            ///< (signed) a > b ? 1 : 0,
 	DSGA_OP_RSUB,           ///< b - a
 	DSGA_OP_STO_NC,         ///< store b into temporary storage, indexed by c. return a
+	DSGA_OP_ABS,            ///< abs(a)
 
 	DSGA_OP_SPECIAL_END,
 };