Reverts inline styles previously applied by updateSlide so the
slider can switch back to single-row mode without leftover height.
Assigns row/column indices and the row-spanning inline styles to
a slide, based on its position in the source list.
Adjusts swiper.virtualSize and snapGrid to fit a multi-row layout
given the per-slide size.
Computes the row/column layout for the given slide list and stores the totals used by
updateSlide/updateWrapperSize.