private boolean isActive( int currentPos ) {
            return array[ currentPos ] != null && array[ currentPos ].isActive;
        }
Return true if currentPos exists and is active.
@param currentPos the result of a call to findPos.
@return true if currentPos is active.
subs/gif-revert/src/main/java/com/engineer/gif/revert/lib/AnimatedGIFWriter.java