Fix bug with incorrect coverage being rendered in jbrowse

[cmdcolin]

I was browsing jbrowse as one does and found a location in a BAM file where the coverage was displayed wrong, approximately doubled, and at "static block" (the jbrowse-2-ism, not bam file format-ism) boundaries

i found that older versions did not have the bug, and bisected to a diff here in jbrowse 2 https://github.com/gmod/jbrowse-components/compare/82618939977da5e71a419~1..82618939977da5e71a419 which led to it being between v1.1.8..v2.0.0 in bam-js https://github.com/gmod/bam-js/compare/v1.1.18..v2.0.0

the cause is fairly technical but a refactoring oversight, where a new "chunk object" (bam file format-ism, representing a range in the file) was not constructed,

the diff was just this to fix

@@ -185,11 +183,10 @@ export default class BAI extends IndexFile {
     // Find chunks in overlapping bins.  Leaf bins (< 4681) are not pruned
     for (const [start, end] of overlappingBins) {
       for (let bin = start; bin <= end; bin++) {
         if (ba.binIndex[bin]) {
           const binChunks = ba.binIndex[bin]
           for (const binChunk of binChunks) {
-            chunks.push(binChunk)
+            chunks.push(new Chunk(binChunk.minv, binChunk.maxv, bin))
           }
         }
       }

my guess is that this bug was somewhat rare but did happen

screenshots

after

before