Cache fontpath

[ffreyer]

I tried to benchmark Makie LaTeXString updates and noticed that joinpath() is taking the majority of the update time.

using Makie, LaTeXStrings, BenchmarkTools
a = L"\sqrt{1-x}"
b = L"\frac{1}{x}"
f,ax,p = text(Point2f(0), text = a);

@benchmark begin
    $p.text = $a
    foreach(kv -> kv[2][], $p.plots[1].attributes.outputs)
    $p.text = $b
    foreach(kv -> kv[2][], $p.plots[1].attributes.outputs)
end

Caching fontname -> fullpath reduces this benchmark from about 1.2ms to 0.27ms.

[ffreyer]

I'm not sure if this change is fine to accept as is. Maybe precompilation can mess something up if paths change? The Dict is empty on reload so I guess that's not a problem.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.43%. Comparing base (efdbaab) to head (3763561).
⚠️ Report is 26 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #157      +/-   ##
==========================================
+ Coverage   79.93%   84.43%   +4.49%     
==========================================
  Files          10       10              
  Lines         658      906     +248     
==========================================
+ Hits          526      765     +239     
- Misses        132      141       +9     

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