function getVisibleContentOffsets() {
        if(isVerticalWritingMode()){
            return {
                top: (options.paginationInfo ? options.paginationInfo.pageOffset : 0)
            };
        }
        return {
            left: (options.paginationInfo ? options.paginationInfo.pageOffset : 0)
                * (isPageProgressionRightToLeft() ? -1 : 1)
        };
    }
@private

Retrieves _current_ offset of a viewport
(related to the beginning of the chapter)

@returns {Object}