Add path conversion, containment and intersection to Region

[JimBobSquarePants]

Prerequisites

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Description

Splits the Region additions out of #415 so they can land on their own, with tests.

New members on Region. They match SkRegion::setPath, SkRegion::contains(const SkRegion&) and SkRegion::intersects(const SkRegion&).

• Region(IPath path, IntersectionRule intersectionRule) scan-converts a path into the integer pixels whose centres lie inside it. Rows and crossings use the same fixed-point rounding as Skia's non-antialiased scan converter.
• Contains(Region region) returns true when every pixel of the argument is in this region.
• Intersects(Region region) returns true when the two regions share at least one pixel.

One change from the #415 code. The integer clip used the ceiling of the path bounds on all four sides. That dropped pixels whose centres lie inside the path when the left or top edge had a fraction of 0.5 or less. The clip now rounds outwards, the same rounding DrawingClipDescriptor applies to clip bounds and the same as SkIRect::roundOut. Seven of the new tests fail with the old clip.

CA1517. The .NET 11 preview SDK reports CA1517 on MoveEdgeBackward, asking for ReadOnlySpan<RegionEdge>. The method writes link fields through the span indexer, and the suggested change gives CS8332 errors, so the rule is suppressed on that method with a pragma.

Tests in RegionTests:

• Path conversion of a zero-area path, an integer rectangle and a fractional rectangle with hand-computed expected pixels.
• A triangle, a concave polygon at positive and negative offsets, and nested rectangles under both fill rules, each checked pixel by pixel against IPath.Contains at pixel centres. Vertex coordinates are chosen so no crossing lands on a pixel centre.
• Contains(Region) and Intersects(Region) on an L shape, empty regions, touching edges and interleaved islands, plus a pixel-membership check on path-built regions.

Run	Result
Region and clip tests, net10.0	98 pass
Region and clip tests, net11.0	98 pass

[codecov]

Codecov Report

❌ Patch coverage is 87.00787% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 79%. Comparing base (f5cc287) to head (57e4991).

Files with missing lines	Patch %	Lines
src/ImageSharp.Drawing/Region.cs	87%	15 Missing and 18 partials ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main    #423    +/-   ##
======================================
  Coverage     79%     79%            
======================================
  Files        232     232            
  Lines      27531   27785   +254     
  Branches    3129    3188    +59     
======================================
+ Hits       21955   22186   +231     
- Misses      4537    4547    +10     
- Partials    1039    1052    +13     

Flag	Coverage Δ
unittests	79% <87%> (+<1%)	⬆️

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