Ensure cache access is thread safe and fix calculated glyph bounds

[JimBobSquarePants]

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• I have written a descriptive pull-request title
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• I have provided test coverage for my change (where applicable)

Description

Fixes #321

[codecov]

Codecov Report

Merging #322 (5a16306) into main (570875b) will increase coverage by 0%.
The diff coverage is 96%.

@@          Coverage Diff          @@
##            main    #322   +/-   ##
=====================================
  Coverage     83%     83%           
=====================================
  Files        224     224           
  Lines      12300   12311   +11     
  Branches    1788    1787    -1     
=====================================
+ Hits       10280   10290   +10     
- Misses      1600    1601    +1     
  Partials     420     420           

Flag	Coverage Δ
unittests	83% <96%> (+<1%)	⬆️

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Impacted Files	Coverage Δ
src/SixLabors.Fonts/StreamFontMetrics.cs	87% <94%> (+<1%)	⬆️
src/SixLabors.Fonts/GlyphPositioningCollection.cs	86% <100%> (+<1%)	⬆️
src/SixLabors.Fonts/TextLayout.cs	87% <100%> (+<1%)	⬆️
src/SixLabors.Fonts/Tables/Cff/Cff1Table.cs	70% <0%> (-6%)	⬇️

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