Fix issue with cube collapsing when 2-d and 3-d datasets were mixed in the image viewer

[astrofrog]

This still needs a changelog entry

[codecov]

Codecov Report

Merging #2188 (1b7858a) into master (538b5f3) will decrease coverage by 0.01%.
The diff coverage is 92.13%.

@@            Coverage Diff             @@
##           master    #2188      +/-   ##
==========================================
- Coverage   88.05%   88.03%   -0.02%     
==========================================
  Files         247      247              
  Lines       23161    23040     -121     
==========================================
- Hits        20394    20284     -110     
+ Misses       2767     2756      -11     

Impacted Files	Coverage Δ
glue/core/fixed_resolution_buffer.py	87.70% <0.00%> (-0.82%)	⬇️
glue/core/link_manager.py	97.64% <ø> (ø)
glue/plugins/coordinate_helpers/deprecated.py	44.44% <0.00%> (ø)
glue/app/qt/application.py	78.57% <50.00%> (ø)
glue/plugins/wcs_autolinking/wcs_autolinking.py	96.75% <95.45%> (ø)
glue/core/coordinates.py	90.62% <100.00%> (ø)
glue/core/data_factories/fits.py	88.88% <100.00%> (ø)
glue/plugins/coordinate_helpers/link_helpers.py	100.00% <100.00%> (ø)
glue/viewers/common/viewer.py	90.83% <100.00%> (ø)
glue/viewers/image/state.py	91.09% <100.00%> (+0.02%)	⬆️
... and 12 more

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