Add a test of the selection code

[rjkroege]

Add a test that demonstrates that selecting a single character is correct along with its necessary dependencies:

• had to add functionality to compute blended image colours
• so rebase the tests for the visually indistinguishable slightly different colours
• add functionality to record fills with arbitrary colours

[codecov[bot]]

Codecov Report

Merging #480 (f5254b9) into master (0f7cef0) will not change coverage. The diff coverage is n/a.

:exclamation: Current head f5254b9 differs from pull request most recent head a9253c3. Consider uploading reports for the commit a9253c3 to get more accurate results

@@           Coverage Diff           @@
##           master     #480   +/-   ##
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  Coverage   58.09%   58.09%           
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  Files          53       53           
  Lines       10676    10676           
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  Hits         6202     6202           
  Misses       4040     4040           
  Partials      434      434           

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