Started adding support for image subsets

[astrofrog]

@Carifio24 this should fix the error you were seeing and adds support for showing subsets for images in WWT:

How the subsets are rendered is not great as we shouldn't be using colormaps but rather single colors, but this might require some changes in PyWWT to make this possible? We could also use fake colormaps.

cc-ing @pkgw for the PyWWT aspect.

[codecov]

Codecov Report

Attention: Patch coverage is 83.78378% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 75.90%. Comparing base (a7a015c) to head (8be5ccf).
Report is 5 commits behind head on main.

Files	Patch %	Lines
glue_wwt/viewer/image_layer.py	66.66%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #94      +/-   ##
==========================================
+ Coverage   69.04%   75.90%   +6.86%     
==========================================
  Files          18       18              
  Lines        1024     1050      +26     
==========================================
+ Hits          707      797      +90     
+ Misses        317      253      -64     

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