Use im->pixelsize consistently

[radarhere]

Suggestion for python-pillow#9975

This seems simpler to me.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (offset-16-fix@41d3cfb). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/libImaging/Offset.c	0.00%	4 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##             offset-16-fix      #28   +/-   ##
================================================
  Coverage                 ?   65.90%           
================================================
  Files                    ?      348           
  Lines                    ?    54789           
  Branches                 ?     3838           
================================================
  Hits                     ?    36106           
  Misses                   ?    18287           
  Partials                 ?      396           

Flag	Coverage Δ
GHA_Docker	65.90% <0.00%> (?)

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[akx]

Thanks, this jogged my brain to re-implement the whole thing in a better way :)