Add ReentrantLocks to library and to each FTFont

[jkrumbiegel]

When people try rendering figures on multiple threads with CairoMakie, I usually see segfaults in FreeType. So this PR adds locks for each FTFont object and for the library, as FreeType is not thread-safe. Makie caches font objects, so multiple figures are bound to use the same font handles. The test I added segfaulted without the locks.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.38%. Comparing base (d66f2aa) to head (f4632ac).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
+ Coverage   95.26%   95.38%   +0.11%     
==========================================
  Files           6        6              
  Lines         317      325       +8     
==========================================
+ Hits          302      310       +8     
  Misses         15       15              

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