Fix export SVG crash on some BigWig tracks

[cmdcolin]

Fixes #3046

Need to make sure to stringify the result. The HTML5 canvas API presumably automatically stringifies the fillStyle object, but canvas2svg needs to do this manually.

It is perhaps a bit surprising that typescript does not catch the Color object as not being a string in our custom fillRect helper function, but this PR should help avoid crash

[codecov]

Codecov Report

Merging #3057 (96c1407) into main (1fbc3c1) will decrease coverage by 0.00%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##             main    #3057      +/-   ##
==========================================
- Coverage   61.29%   61.28%   -0.01%     
==========================================
  Files         591      591              
  Lines       26981    26981              
  Branches     6527     6527              
==========================================
- Hits        16537    16536       -1     
  Misses      10146    10146              
- Partials      298      299       +1     

Impacted Files	Coverage Δ
...lugins/wiggle/src/XYPlotRenderer/XYPlotRenderer.ts	66.29% <33.33%> (ø)
...quenceRenderer/components/DivSequenceRendering.tsx	92.55% <0.00%> (-2.13%)	⬇️
products/jbrowse-web/src/util.ts	27.27% <0.00%> (ø)
plugins/breakpoint-split-view/src/model.ts	78.26% <0.00%> (+1.08%)	⬆️

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