Fix #227 -- Always warn about missing dimension fields

[jmsmkn]

At least the width dimension is always needed to generate the full source set, regardless of the requested aspect ratio. Since only smaller versions of the original file are created, the original size is the upper bound for the source set.

Close #227

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (0ec2e4d) to head (81803ed).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #229   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines          494       494           
=========================================
  Hits           494       494           

Flag	Coverage Δ
celery	85.02% <100.00%> (ø)
cleanup	88.46% <100.00%> (ø)
dj5.1	88.46% <100.00%> (ø)
dj5.2	88.46% <100.00%> (ø)
django-rq	85.02% <100.00%> (ø)
dramatiq	85.02% <100.00%> (ø)
drf	96.35% <100.00%> (ø)
macos-latest	88.66% <100.00%> (ø)
py3.10	88.46% <100.00%> (ø)
py3.12	88.46% <100.00%> (ø)
py3.13	88.46% <100.00%> (ø)
py3.9	88.46% <100.00%> (ø)
ubuntu-latest	88.46% <100.00%> (ø)
windows-latest	81.58% <ø> (ø)

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[codingjoe]

I updated the PR message, since this will come as a somewhat breaking change to users. I also updated the documentation in accordance with the fields always being required.