Fix indexer offset for post positioning methods

[JimBobSquarePants]

Prerequisites

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• I have provided test coverage for my change (where applicable)

Description

We were failing to offset the lookups for fixing up cursive and mark attachments. Fixes #300

[codecov]

Codecov Report

Merging #301 (c0ae03e) into main (ca3231c) will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##            main    #301   +/-   ##
=====================================
  Coverage     83%     83%           
=====================================
  Files        222     222           
  Lines      12276   12276           
  Branches    1781    1781           
=====================================
  Hits       10253   10253           
  Misses      1591    1591           
  Partials     432     432           

Flag	Coverage Δ
unittests	83% <100%> (ø)

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Impacted Files	Coverage Δ
...bors.Fonts/Tables/AdvancedTypographic/GPosTable.cs	84% <100%> (ø)

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