Fix interpreter issues and prevent multple kerning operations

[JimBobSquarePants]

Prerequisites

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Description

This pull request introduces several important improvements and bug fixes to the font rendering engine, focusing on glyph positioning, kerning, and TrueType hinting. The changes enhance correctness, efficiency, and maintainability, particularly in the handling of glyph state and the implementation of the TrueType interpreter. Additionally, some code modernizations and new test reference images are included.

Glyph Positioning and Kerning Improvements

• Added explicit tracking of glyph positioning (IsPositioned) and kerning (IsKerned) states in GlyphShapingData, preventing redundant processing and ensuring accurate state propagation during glyph shaping and kerning. The kerning logic now checks and sets IsKerned to avoid double application.
• Optimized glyph positioning updates in TextLayout by avoiding redundant updates for consecutive TextRuns using the same font.

TrueType Interpreter Bug Fixes and Enhancements

• Corrected several subtle bugs in the TrueTypeInterpreter, including:
  • Proper toggling of the OnCurve flag for the correct point index.
  • Correctly setting the iupXCalled flag in the IUP instruction.
  • Fixed logic for determining if any zone pointer is in the twilight zone.
  • Corrected the use of zone pointers in vector calculations.
• Completely reimplemented the Round function to precisely match FreeType v40's rounding semantics, ensuring correct and consistent rendering across all rounding modes.

Documentation and Code Modernization

• Expanded and clarified the documentation for the TrueTypeInterpreter, detailing the reference model and behavioral expectations for modern and legacy fonts.
• Modernized array initialization and usage of collection expressions for improved readability and performance in kerning table loading code.

Testing

• Added new reference output images for hinting robustness tests using Arial, OpenSans, and Tahoma to validate rendering changes.

[Copilot]

Pull request overview

This pull request addresses critical bugs in the TrueType interpreter and introduces state tracking for glyph positioning and kerning operations to prevent redundant processing. The changes improve font rendering accuracy and performance while modernizing the codebase with collection expressions.

• Fixed multiple bugs in the TrueType interpreter including incorrect point index toggling, zone pointer usage, and IUP flag setting
• Completely reimplemented the Round function to match FreeType v40's rounding semantics across all modes
• Added IsPositioned and IsKerned flags to prevent double application of positioning and kerning operations

Reviewed changes

Copilot reviewed 15 out of 16 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/SixLabors.Fonts.Tests/TextLayoutTestUtilities.cs	Added customDecorations parameter to control test decoration pens conditionally
tests/SixLabors.Fonts.Tests/TestFonts.cs	Added Tahoma font path for hinting tests
tests/SixLabors.Fonts.Tests/Issues/Issues_462.cs	Updated test calls to pass customDecorations: true
tests/SixLabors.Fonts.Tests/ImageComparison/TestImageExtensions.cs	Added PNG encoder with optimal compression settings for test output
tests/SixLabors.Fonts.Tests/HintingTests.cs	New robustness tests for TrueType hinting with Arial, Tahoma, and OpenSans
tests/Images/ReferenceOutput/*.png	Reference output images for hinting tests (Git LFS tracked)
src/SixLabors.Fonts/TextLayout.cs	Optimized to skip redundant UpdatePositions calls for consecutive runs with same font
src/SixLabors.Fonts/Tables/TrueType/Hinting/TrueTypeInterpreter.cs	Fixed bugs in OnCurve toggling, IUP flag setting, zone pointer usage, and twilight zone detection; reimplemented Round function
src/SixLabors.Fonts/Tables/General/Kern/KerningTable.cs	Added kerning state check and modernized with collection expressions
src/SixLabors.Fonts/Tables/General/Kern/KerningSubTable.cs	Removed var keyword for explicit type declaration
src/SixLabors.Fonts/Tables/General/Kern/Format0SubTable.cs	Improved comment formatting and removed var keyword
src/SixLabors.Fonts/GlyphShapingData.cs	Added IsPositioned and IsKerned properties with copy constructor support
src/SixLabors.Fonts/GlyphPositioningCollection.cs	Integrated IsPositioned flag setting and checking to prevent redundant processing

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