Fix Knife Tool on merging different shapes

Agata Cackorequested to merge
tymond/krita:tiar/fix_knife_tool_incorrect_results into master

Before this commit there was a mistake cutting the index short when appending edges, which resulted in incorrect results every time a user tried to remove a gutter in between two different shapes. This also caused broken unit tests. This commit fixes that by not forcing the end index in appending to be lower than the segments count, because the for loop will never reach that index anyway, so it's not a needed sanity check. The state before on the other hand prevented appending the last edge.

It also cleans up unit tests and adds a few more cases, and results.

Test Plan

(Tell us how to test the changes you made.)

Formalities Checklist

  • I confirmed this builds.
  • I confirmed Krita ran and the relevant functions work.
  • I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
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  • I made sure my code conforms to the standards set in the HACKING file.
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  • Does the patch add a user-visible feature? If yes, is there a documentation MR ready for it at Krita Documentation Repository?

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