Fix 355

[JimBobSquarePants]

Prerequisites

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

Description

Fixes #355

TR50 vertical codepoint substitution should only take place when the layout is not horizontal.

[codecov]

Codecov Report

Merging #356 (b10de71) into release/v1.x (b532dee) will increase coverage by 0%.
Report is 1 commits behind head on release/v1.x.
The diff coverage is 100%.

@@             Coverage Diff              @@
##           release/v1.x    #356   +/-   ##
============================================
  Coverage            86%     86%           
============================================
  Files               238     238           
  Lines             15187   15189    +2     
  Branches           2110    2108    -2     
============================================
+ Hits              13071   13073    +2     
  Misses             1624    1624           
  Partials            492     492           

Flag	Coverage Δ
unittests	86% <100%> (+<1%)	⬆️

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

Files Changed	Coverage Δ
src/SixLabors.Fonts/TextLayout.cs	86% <100%> (+<1%)	⬆️

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