Fix pileup "sort by" setting being lost on zoom level change

[cmdcolin]

Fixes #3318

The BaseLinearDisplayModel has a currBpPerPx that collided with the LinearPileupDisplay's currBpPerPx. Renamed to currSortBpPerPx to disambiguate

[codecov]

Codecov Report

Merging #3319 (638bfca) into main (d54632d) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #3319      +/-   ##
==========================================
+ Coverage   59.44%   59.51%   +0.06%     
==========================================
  Files         736      736              
  Lines       28997    28999       +2     
  Branches     7017     7017              
==========================================
+ Hits        17238    17259      +21     
+ Misses      11513    11494      -19     
  Partials      246      246              

Impacted Files	Coverage Δ
...lugins/alignments/src/LinearPileupDisplay/model.ts	66.39% <100.00%> (-0.56%)	⬇️
...s/alignments/src/PileupRenderer/PileupRenderer.tsx	51.01% <0.00%> (-0.61%)	⬇️
products/jbrowse-web/src/util.ts	48.33% <0.00%> (ø)
packages/core/util/layouts/GranularRectLayout.ts	90.90% <0.00%> (+2.16%)	⬆️
plugins/hic/src/HicRenderer/HicRenderer.tsx	100.00% <0.00%> (+4.54%)	⬆️
plugins/alignments/src/PileupRenderer/sortUtil.ts	62.29% <0.00%> (+27.86%)	⬆️

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[cmdcolin]

adds test for the zoom out now also