when updating active slide, when position is not provided, use the index of the last active slide instead of the css translate value

this is to resolve an issue which adding a page after the active slide would result in a change in active index during quick swiping. adding a new page cases swiper to call reInit and recalculate the slides using the css translate value instead of a specific position.

during quick swiping in addition to appending and removing slides, the css translate value is not a reliable indicator for the active slide.