Convert many global variables involving vectors to use Ref instead.

[joa-quim]

Following a Claude analysis.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.14035% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.20%. Comparing base (79d24b0) to head (e024cb8).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/imshow.jl	0.00%	3 Missing ⚠️
src/common_options.jl	98.14%	2 Missing ⚠️
src/psxy.jl	92.85%	2 Missing ⚠️
src/makecpt.jl	75.00%	1 Missing ⚠️
src/potential/gmtgravmag3d.jl	0.00%	1 Missing ⚠️
src/subplot.jl	90.90%	1 Missing ⚠️
src/utils_project.jl	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1841      +/-   ##
==========================================
- Coverage   85.22%   85.20%   -0.03%     
==========================================
  Files         176      176              
  Lines       26420    26420              
==========================================
- Hits        22517    22510       -7     
- Misses       3903     3910       +7     

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