`DISTPREV` of VCF should be calculated considering nearby tiles, not within single tile

[sehilyi]

Gosling calculates the distance to previous mutations (DISTPREV) for VCF files. However, such a calculation only occurs within a tile. (Gosling divides a visible scene into multiple tiles when loading data.) This leads to incorrect calculation.

For example, the first point mutation in a tile does not respect the last point mutation from the previous tile. This results in null of DISTPREV. The first and the last point mutations in a tile should look up mutations in the previous and next tiles, respectively. cc @dominikglodzikhms @crfmc

gosling.js/src/data-fetchers/vcf/vcf-worker.ts

Line 234 in 5fe5721

const highPriority = output.sort((a, b) => -(a.DISTPREV ?? 0) + (b.DISTPREV ?? 0)).slice(0, sampleLength / 2.0);