Fix new TextMeasurer Bounds calculations so origin is absolute.

[JimBobSquarePants]

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

Description

This pull request enhances the documentation and correctness of text measurement methods in TextMeasurer.cs, clarifying the distinction between logical advance and rendered glyph bounds, and ensuring absolute coordinates are used for layout calculations. It also updates the implementation to properly union the logical advance with the rendered bounds, and improves per-character measurement accuracy. The test utility is updated to reflect these changes.

Documentation improvements:

• Added detailed remarks to all public measurement methods in TextMeasurer.cs, explaining the differences between logical advance, ink bounds, and renderable bounds, and referencing related methods for clarity. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

Measurement logic corrections:

• Updated MeasureRenderableBounds(ReadOnlySpan<char>, TextOptions) to union the absolute advance rectangle (with origin) with the rendered bounds, ensuring the returned rectangle is in absolute coordinates and contains both logical and ink bounds.
• Refined per-character renderable bounds calculation in TryGetCharacterRenderableBounds to use absolute advance rectangles and union them with glyph ink bounds, improving accuracy for layout and rendering.

Test utility update:

• Modified TestLayout in TextLayoutTestUtilities.cs to calculate width and height using the new measurement logic, ensuring the test reflects the updated bounding box calculation.

[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 83%. Comparing base (3214ad7) to head (ddead05).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/SixLabors.Fonts/TextMeasurer.cs	40%	3 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #507   +/-   ##
=====================================
- Coverage     83%     83%   -1%     
=====================================
  Files        274     274           
  Lines      19167   19169    +2     
  Branches    2714    2714           
=====================================
+ Hits       16048   16049    +1     
- Misses      2441    2442    +1     
  Partials     678     678           

Flag	Coverage Δ
unittests	83% <40%> (-1%)	⬇️

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