Visualize single channel 8 bit image data

[iche033]

Signed-off-by: Ian Chen ichen@osrfoundation.org

Summary

Extended Image Display plugin to support visualizing single channel 8 bit unsigned integer data in a similar way to the 16 bit data. This is used by gazebosim/gz-sim#614 to visualize 8 bit thermal image data (see PR for screenshot)

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)

Note to maintainers: Remember to use Squash-Merge

[codecov]

Codecov Report

Merging #176 (7294517) into ign-gui4 (1190ed0) will decrease coverage by 0.80%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##           ign-gui4     #176      +/-   ##
============================================
- Coverage     61.57%   60.77%   -0.81%     
============================================
  Files            21       21              
  Lines          2803     2840      +37     
============================================
  Hits           1726     1726              
- Misses         1077     1114      +37     

Impacted Files	Coverage Δ
src/plugins/image_display/ImageDisplay.cc	18.87% <0.00%> (-4.40%)	⬇️

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[adlarkin]

I think this is good to go - do we need to add some sort of test?

[adlarkin]

Since there are currently no tests for pixel formats with image display (https://github.com/ignitionrobotics/ign-gui/blob/a60cb7d61339b407ff3285ddbf16f618c4c4b3b9/src/plugins/image_display/ImageDisplay_TEST.cc), I think that this is fine as is. Perhaps we can add some tests later if people think that would be a worthwhile contribution.