Add ability to render only snpcoverage in jbrowse-img

[cmdcolin]

example usage

jb2export --fasta file.fa --bam file.bam snpcov height:1000

renders just the snpcov

requested on office hours

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (main@cdd6716). Click here to learn what that means.

❗ Current head ca3e30f differs from pull request most recent head 2719fed. Consider uploading reports for the commit 2719fed to get more accurate results

Files	Patch %	Lines
products/jbrowse-img/src/renderRegion.tsx	75.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4219   +/-   ##
=======================================
  Coverage        ?   63.00%           
=======================================
  Files           ?     1071           
  Lines           ?    31207           
  Branches        ?     7461           
=======================================
  Hits            ?    19663           
  Misses          ?    11369           
  Partials        ?      175           

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