Ensure cache access is thread safe and fix calculated glyph bounds

[JimBobSquarePants]

Prerequisites

• [x] I have written a descriptive pull-request title
• [x] I have verified that there are no overlapping pull-requests open
• [x] I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules :cop:.
• [x] I have provided test coverage for my change (where applicable)

Description

Fixes #321

[codecov[bot]]

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%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

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

:mega: We’re building smart automated test selection to slash your CI/CD build times. Learn more