gr: restore transparency after drawing axes

[jheinen]

fixes https://discourse.julialang.org/t/plots-gr-savefig-adds-transparency-to-my-picture/93511

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.05 ⚠️

Comparison is base (c16a009) 90.31% compared to head (7948665) 90.27%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4724      +/-   ##
==========================================
- Coverage   90.31%   90.27%   -0.05%     
==========================================
  Files          40       40              
  Lines        8728     8729       +1     
==========================================
- Hits         7883     7880       -3     
- Misses        845      849       +4     

Impacted Files	Coverage Δ
src/backends/gr.jl	90.47% <100.00%> (+<0.01%)	⬆️

... and 2 files with indirect coverage changes

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