use Ref() instead of pointer() for output arguments

[jwahlstrand]

I think Ref is the recommended way to use output arguments: https://docs.julialang.org/en/v1/base/c/#Base.pointer

Might be a fix for #363 Never mind, I was able to check this locally and it doesn't fix the issue. It's definitely related to the recent Cairo_jll upgrade.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.02%. Comparing base (ba5afd5) to head (dc0158a).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #364      +/-   ##
==========================================
- Coverage   92.52%   92.02%   -0.51%     
==========================================
  Files           1        1              
  Lines         589      552      -37     
==========================================
- Hits          545      508      -37     
  Misses         44       44              

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