Rework aliased rasterization and path lowering to allow full hinting

[JimBobSquarePants]

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Description

Fix #134

Replace threshold-based aliased coverage with exact center-sampled rasterization in both the CPU and WebGPU backends, including profile-driven thin-feature recovery for fills and updated hard-clip behavior. Rename the WebGPU flatten stage to path lowering, add the new profile/tile scene data plumbing, and update related scheduling, docs, and shader bindings. The change also removes antialias-threshold plumbing, adds shared linear-geometry profile storage, refreshes text and aliasing tests/reference images, and bumps SixLabors.Fonts.

Pipeline terminology and documentation updates:

• Renamed the "flatten" stage and related identifiers to "path lowering" throughout the C# and WGSL shader code, including class and file names, comments, and error messages. This provides clearer terminology and aligns with upstream projects. (src/ImageSharp.Drawing.WebGPU/Shaders/FlattenComputeShader.cs → PathLoweringComputeShader.cs, [1] [2] [3] [4]; [5] [6] [7]; [8]; [9] [10] [11]; [12] [13]; [14]; [15]
• Updated documentation comments in several shaders to clarify the use of "final lines" instead of "flattened lines" and to better describe the data flow between stages. [1] [2] [3] [4] [5]

Shader and pipeline binding expansions:

• Added new bind group layout entries for profile data and path bounding boxes to support advanced rendering features, updating both the C# and WGSL code to match. (FineAreaComputeShader.cs, [1] [2]; PathRowAllocComputeShader.cs, [3]

Shader structure and field updates:

• Updated the PathBbox WGSL struct and related documentation to use coverage_data instead of coverage_threshold, reflecting a more general approach to coverage adjustment. [1] [2]

Configuration and shared resources:

• Extended the Config WGSL struct to include offsets for segment-profile tag and record tables, supporting new features in the rendering pipeline.

Project and solution file maintenance:

• Removed an unused shared project import from ImageSharp.Drawing.WebGPU.csproj and cleaned up the solution file. [1] [2]

[codecov]

Codecov Report

❌ Patch coverage is 96.57588% with 44 lines in your changes missing coverage. Please review.
✅ Project coverage is 79%. Comparing base (d8c0e28) to head (f1fa7c7).

Files with missing lines	Patch %	Lines
...eSharp.Drawing/Processing/DrawingClipDescriptor.cs	55%	8 Missing ⚠️
...p.Drawing/Processing/Backends/DefaultRasterizer.cs	97%	2 Missing and 5 partials ⚠️
.../ImageSharp.Drawing.WebGPU/WebGPUSceneResources.cs	86%	6 Missing ⚠️
...c/ImageSharp.Drawing.WebGPU/WebGPUSceneDispatch.cs	82%	2 Missing and 3 partials ⚠️
...ageSharp.Drawing/Processing/Backends/FlushScene.cs	95%	4 Missing and 1 partial ⚠️
src/ImageSharp.Drawing/LinearGeometryProfiles.cs	97%	2 Missing and 2 partials ⚠️
...rocessing/Backends/DefaultRasterizer.Linearizer.cs	96%	2 Missing and 1 partial ⚠️
...rc/ImageSharp.Drawing.WebGPU/WebGPUSceneEncoder.cs	99%	0 Missing and 2 partials ⚠️
...ocessing/Backends/DefaultDrawingBackend.Helpers.cs	85%	1 Missing and 1 partial ⚠️
...rawing.WebGPU/Shaders/PathLoweringComputeShader.cs	50%	1 Missing ⚠️
... and 1 more

Additional details and impacted files

@@          Coverage Diff           @@
##            main    #411    +/-   ##
======================================
  Coverage     79%     79%            
======================================
  Files        231     232     +1     
  Lines      26195   27005   +810     
  Branches    2953    3086   +133     
======================================
+ Hits       20746   21542   +796     
- Misses      4567    4574     +7     
- Partials     882     889     +7     

Flag	Coverage Δ
unittests	79% <96%> (+<1%)	⬆️

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