Linelevel: fix the difference option

[markotoplak]

pySNOM returns a smaller image in this case, thus there were problems.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 89.98%. Comparing base (b0444ce) to head (3b5f229).
Report is 76 commits behind head on master.

Files with missing lines	Patch %	Lines
...ngecontrib/snom/widgets/preprocessors/linelevel.py	80.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (b0444ce) and HEAD (3b5f229). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (b0444ce)	HEAD (3b5f229)
	2	1

Additional details and impacted files

@@             Coverage Diff              @@
##            master      #30       +/-   ##
============================================
- Coverage   100.00%   89.98%   -10.02%     
============================================
  Files            4       16       +12     
  Lines           34      689      +655     
============================================
+ Hits            34      620      +586     
- Misses           0       69       +69     

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