fixed range calculation for lazyLoad

center mode requires different range calculation as slidesToShow/2 slides are displayed before and after the current slide.

this should fix issue kenwheeler/slick#358 and also implements a fixed range-calculation for the "fade" option that preloads the previous and next image (as previously resolved in issue kenwheeler/slick#186 / pull-request kenwheeler/slick#219 by simply making it behave like the "center" option)