Add PolygonStroker and RemoveSelfIntersections for robust, high-performance stroke normalization

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

This PR adds stroking and normalization to SixLabors.PolygonClipper for handling non-simple, real-world geometry.

What’s Included

• PolygonClipper.Normalize(Polygon)
  New explicit normalization API that resolves self-intersections and overlaps into canonical positive-winding output.
• PolygonStroker + StrokeOptions
  New stroke-geometry pipeline for polygons/contours, with configurable join/cap behavior and optional output normalization.

Behavior and Compatibility

• Normalization is explicit and opt-in via Normalize(...).
• Existing boolean operation behavior is unchanged.
• Stroker output can be normalized before return, so callers can get stable, fillable geometry without extra cleanup steps.

Why This Matters

Many production inputs are not simple polygons (self-intersections, touching edges, compound paths), especially from text outlines and imported vector artwork. This pipeline provides a practical preprocessing/geometry-generation path that improves robustness for downstream operations.

Validation and Docs

• Tests were added/updated for complex scenarios (including compound and glyph-like inputs).
• README/docs were updated to describe stroking and normalization usage and semantics.

Performance Note

In the ImageSharp.Drawing DrawPolygonMediumThick benchmark, replacing the default stroker/clipper path with this implementation reduced runtime from ~14 ms to ~3 ms (about 79% faster) and also reduced allocation pressure.

CC @stefannikolei

[stefannikolei]

I like the third Party notices 👍

[JimBobSquarePants]

OK. I'm merging this. Super happy with the result!