Exit process after rendering to speed up jb2export

[cmdcolin]

Currently running jb2export will finish quickly and print out "Finished rendering 1.24s" for example, but the process will stay running. This may be due to an autorun or something keeping nodejs alive.

We could try to dig into why it is kept alive, but doing a process.exit after the image is generated seems reasonable also

This would fix an older issue open at GMOD/jb2export#6

[codecov]

Codecov Report

Merging #2799 (1199112) into main (5fc0460) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

❗ Current head 1199112 differs from pull request most recent head 8cb0ed5. Consider uploading reports for the commit 8cb0ed5 to get more accurate results

@@            Coverage Diff             @@
##             main    #2799      +/-   ##
==========================================
- Coverage   59.97%   59.96%   -0.02%     
==========================================
  Files         584      584              
  Lines       26658    26665       +7     
  Branches     6450     6451       +1     
==========================================
  Hits        15989    15989              
- Misses      10341    10347       +6     
- Partials      328      329       +1     

Impacted Files	Coverage Δ
products/jbrowse-img/src/index.js	0.00% <0.00%> (ø)

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