Fix centerInsufficientSlides with slidesOffsetBefore/After in v12

Summary

In Swiper v12.0.0+, centerInsufficientSlides under-centers slides when slidesOffsetBefore and/or slidesOffsetAfter are set. The wrapper ends up shifted by (offsetBefore + offsetAfter) / 2 compared to v11 behavior. This PR fixes the centering math so offsets are applied exactly once.

Root cause

Starting in v12 (commit 74cc297), updateSlides() computes:

• swiperSize = swiper.size - offsetBefore - offsetAfter

But the centerInsufficientSlides block still treated offsets as not accounted for and subtracted them again via offsetSize, effectively shrinking the available size by 2 * (offsetBefore + offsetAfter).

Reproduction

Minimal config:

new Swiper('.swiper', {
  slidesPerView: 'auto',
  spaceBetween: 12,
  centerInsufficientSlides: true,
  slidesOffsetBefore: 48,
  slidesOffsetAfter: 48,
});

Verification

• With fewer slides than needed to fill the container, v12 now centers the same as v11 when offsets are configured.
• No behavior change when offsets are 0 / unset.