Fix TrueTypeInterpreter pool state pollution

[JimBobSquarePants]

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Description

Replaces #540

@christianrondeau I couldn't push to your fork (I think because you are an org) so I replicated your commit here to ensure you have co-ownership.

This pull request improves the determinism and correctness of the TrueType hinting engine by ensuring that the interpreter's state is fully reset between renders, preventing non-deterministic output when reusing interpreters from a pool. It also improves CVT scaling precision and adds a regression test to guarantee consistent results. The most important changes are:

TrueType Interpreter State Management:

• Updated SetControlValueTable in TrueTypeInterpreter.cs to always run the prep program even if the CVT table is missing, and to fully reset interpreter state (twilight zone, storage, rounding, zone pointers, etc.) before each prep execution, matching FreeType's behavior and preventing history-dependent, non-deterministic hinting results. [1] [2]

Hinting Precision:

• Improved CVT value scaling to use 26.6 fixed-point rounding, matching FreeType and avoiding subtle rounding errors that could affect glyph rendering.

Testing and Robustness:

• Added a regression test Hinting_OutputIsIndependentOfPreviouslyRenderedSizes in HintingTests.cs to ensure that rendering output is independent of previously rendered sizes and interpreter reuse. [1] [2]
• Updated the reference output image for the hinting robustness test to reflect the corrected, deterministic rendering.

[codecov]

Codecov Report

❌ Patch coverage is 89.47368% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 84%. Comparing base (0d7e223) to head (ae7e3aa).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...nts/Tables/TrueType/Hinting/TrueTypeInterpreter.cs	89%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #541   +/-   ##
=====================================
  Coverage     84%     84%           
=====================================
  Files        332     332           
  Lines      24668   24684   +16     
  Branches    3611    3612    +1     
=====================================
+ Hits       20771   20785   +14     
- Misses      3010    3011    +1     
- Partials     887     888    +1     

Flag	Coverage Δ
unittests	84% <89%> (+<1%)	⬆️

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